How can I still not get in if my waiver has been approved?

AR 40-501 states

3–6. Eyes
The causes for referral to the DES are as follows:
a. Active eye disease or any progressive organic disease or degeneration. Regardless of the stage of activity, when
resistant to treatment and affects the distant visual acuity or visual fields so that distant visual acuity does not meet the standard stated in paragraph 3–7e or the diameter of the field of vision in the better eye is less than 20 degrees.
b. Aphakia. Bilateral.
c. Atrophy of the optic nerve. Due to disease when vision does not meet the standards of paragraphs 3–7e or 3–7f. d. Glaucoma. If resistant to treatment or affecting visual fields as in paragraph 3–6a, or if side effects of required medication are functionally incapacitating.
e. Degenerations. When vision does not meet the standards of paragraph 3–7, or when vision is correctable only by the use of contact lenses or other special corrective devices (telescopic lenses and so forth.).
f. Diseases and infections of the eye. When chronic, more than mildly symptomatic, progressive, and resistant to treat- ment after a reasonable period. This includes intractable allergic conjunctivitis inadequately controlled by medications and immunotherapy or dry eye inadequately controlled with medical or surgical intervention.
g. Residuals or complications of injury, disease, or surgery. Postoperatively uncomplicated photorefractive keratec- tomy (PRK), laser epithelial keratomileusis (LASEK), or laser-assisted in situ keratomileusis (LASIK) when progressive or when reduced visual acuity does not meet the criteria stated in paragraph 3–7 (see DA Pam 40–502 for profiling in- structions).
h. Unilateral detachment of retina. If any of the following exists:
(1) Visual acuity does not meet the standard stated in paragraph 3–7e.
(2) The visual field in the better eye is constricted to less than 20 degrees.
(3) Uncorrectable diplopia exists.
(4) Detachment results from organic progressive disease or new growth, regardless of the condition of the better eye. i. Bilateral detachment of retina. Regardless of etiology or results of corrective surgery.
j. An eye has been enucleated.

DoD 6130.03 section 5: Disqualifying conditions

5.3. EYES. a. Lids.
(1) Current symptomatic blepharitis.
(2) Current blepharospasm.
(3) Current dacryocystitis, acute or chronic.
(4) Defect or deformity of the lids or other disorders affecting eyelid function, including ptosis, sufficient to interfere with vision, require head posturing, or impair protection of the eye from exposure.
(5) Current growths or tumors of the eyelid, other than small, non-progressive, asymptomatic, benign lesions.
b. Conjunctiva.
(1) Current acute or chronic conjunctivitis excluding seasonal allergic conjunctivitis.
(2) Current pterygium if condition encroaches on the cornea in excess of 3 millimeters (mm), is symptomatic, interferes with vision, or is progressive.
(3) History of pterygium recurrence after any prior surgical removal.
c. Cornea.
(1) Corneal dystrophy or degeneration of any type, including but not limited to keratoconus of any degree.
(2) History of any incisional corneal surgery including, but not limited to, partial or full thickness corneal transplant, radial keratotomy, astigmatic keratotomy, or corneal implants (e.g., Intacs®).
(3) Corneal refractive surgery performed with an excimer or femtosecond laser, including but not limited to photorefractive keratectomy, laser epithelial keratomileusis, laser- assisted in situ keratomileusis, and small incision lenticule extraction, if any of the following conditions are met:
(a) Pre-surgical refractive error in either eye exceeded a spherical equivalent of +8.00 or -8.00 diopters.
(b) Pre-surgical astigmatism exceeded 3.00 diopters.
(c) Within 180 days of accession medical examination.
(d) Complications, ongoing medications, ophthalmic solutions, or any other therapeutic interventions required beyond 180 days of procedure.
(e) Post-surgical refraction in each eye is not stable as demonstrated by at least two separate refractions at least 1 month apart, with initial refraction at least 90 days post-procedure, and the most recent of which demonstrates either more than +/- 0.50 diopters difference for spherical vision or more than +/- 0.50 diopters for cylinder vision.
(4) Current or recurrent keratitis.
(5) History of herpes simplex virus keratitis.
(6) Current corneal neovascularization, unspecified, or corneal opacification from any cause that is progressive or reduces vision.
(7) Any history of uveitis or iridocyclitis.
d. Retina. Any history of any abnormality of the retina, choroid, or vitreous. e. Optic Nerve.
(1) Any history of optic nerve disease, including but not limited to optic nerve inflammation, optic nerve swelling, or optic nerve atrophy.
(2) Any optic nerve anomaly. f. Lens.
(1) Current aphakia, history of lens implant to include implantable collamer lens, or any history of dislocation of a lens.
(2) Any history of opacities of the lens, including cataract.
g. Ocular Mobility and Motility.
(1) Current or recurrent diplopia.
(2) Current nystagmus other than physiologic “end-point nystagmus.” (3) Esotropia, exotropia, and hypertropia.
(4) History of restrictive ophthalmopathies.
h. Miscellaneous Defects and Diseases.
(1) History of abnormal visual fields.
(2) Absence of an eye.
(3) History of disorders of globe.
(4) Current unilateral or bilateral exophthalmoses.
(5) History of glaucoma, ocular hypertension, pre-glaucoma, or glaucoma suspect. (6) Any abnormal pupillary reaction to light or accommodation.
(7) Asymmetry of pupil size greater than 2 mm.
(8) Current night blindness.
(9) History of intraocular foreign body, or current corneal foreign body.
(10) History of ocular tumors.
(11) History of any abnormality of the eye or adnexa, not specified in Paragraphs 5.3.h.(1)-(10), which threatens vision or visual function.
5.4. VISION.
a. Current distant visual acuity of any degree that does not correct with spectacle lenses to at least 20/40 in each eye. b. For entrance into Service academies and officer programs, the individual DoD Components may set additional requirements. The DoD Components will determine special administrative criteria for assignment to certain specialties.
c. Current near visual acuity of any degree that does not correct to 20/40 in the better eye.
d. Current refractive error (hyperopia, myopia, astigmatism) in excess of -8.00 or +8.00 diopters spherical equivalent or astigmatism in excess of 3.00 diopters.
e. Any condition that specifically requires contact lenses for adequate correction of vision, such as corneal scars and opacities and irregular astigmatism.
f. Color vision requirements will be set by the individual DoD Components.

The short answer is yes. You could get denied entry if the MEPS doctor feels and can moderately prove that in their opinion and expert medical knowledge you do not meet the standard for vision for the military.