How does an E6 have 6 service stripes?

Could be prior service, changed over from another branch

PFC O'Hara,
You answered your own question. Indeed, it would appear you have read AR-670-1 to know that one service stripe represents 3 years of service and it is good to know you have the ablity to multiply or at least add, but you don't appear to possess knowledge of how promotions to the higher ranks works. Certain MOS's have low turn over at the higher ranks, it could be there aren't anymore slots for him/her. She/he may have done something in his/her past that is keeping. Her /him from getting promoted, but you don't know that. What you do know is that he/she out ranks you and deserves your respect, not questioning why she/he has the rank they do.

If I had to guess.. the answer would be in the right side of his uniform... Although very blurry.. Prior service in another branch.. and only wearing the three highest ribbons he could have been deployed to combat in that Branch also. Which could also mean he took a reduction in rank to switch branches.. which throws out Dirtag or under achiever. In fact I bet we would all be impressed how fast he moved up in rank after switching branches. :)

If you think an E6 is a poser, why not engage him in a friendly manner and ask about his past; what jobs have you had, where were you stationed, were you "in country" and how many times? These questions may alleviate your angst about something that is irrelevent. Check out his chest as well and read what he has done. You may be barking up the wrong tree and this quiet observation may answer all your questions.

prior service could be the answer as well. or service in another branch. look closely at the RIGHT sleeve as well for combat area service. also look at the chest board , blow up the picture... there are some awards there he may be authorized to wear that are NOT army. look closely my friends... dont assume.

He may have gotten out for a time, and now is back. You get back and can wear all you earned…

Was gonna say probably an NJP but that doesn't mean he or she is a POS in any way. It's possible for a first class petty officer to have six *gold* stripes with or without an NJP. Maybe they screwed up early on and then went 18 years with a stellar career after that. Maybe they knew doing the right thing would get them sent to Captain's Mast but did it anyway.
When I got out of the Navy, 3 years' good conduct earned you a medal. 12 years' good conduct earned you gold stripes. 4 years service got you a service stripe and at 20 you either had to be in a critical rate, made E7, or retire.
NJP doesn't always equal POS, nor does 24 years at E-6 or below. It's also possible to be in a critical rate at 20 years, with good conduct you could easily get high year tenure waived if that's the case.
Bottom line is this: keep your nose in your own business. Treat the dude how you wanna be treated, even the saltiest E-6 will respect that.

Can't speak for the Army, I wasn't in the Army and my two uncle that retired from the Army never talked about promotions, although I believe I remember one saying something. He was Transportation and at Ft. Eustis. He was an SFC and I believe got promoted before he retired, ben a long time as he retired in '58. Anyway, I went into the Coast Guard right out of High School in '65. My first Duty station was about 3.5 miles from Otis AFB. At that time the retention rate in the AF wasn't all that hot due to lack of promotion. As I said, I can't speak to the Army, but the Air Force had these old-time upper-level NCOs that found a home. There were X number of Billets and unless someone at the top retired, then no Billet opened so the system was stagnant and thus NO promotions. I think in the end the AF ended up creating more billets. With the Coast Guard, you had your Rate / MOS. Let's say Engineman (Now Machinery Technician). You come out of Basic as an E-2. You have to take and pass a Correspondence Course for E-3 (Seaman, Fireman, or Airman) then pass your Practical Factors. After that, you have two ways to go to reach E4. The same route as you took to get from E-2 to E-3, but with something new added, Service wide Exams. Here you are competing with other across the service to fill, say, 20 open billets. If you make the list, you will draw Pro Pay for that rate and will be promoted when a billet opens. Now, if you are not promoted before the next Service Wide Exam, you have to take it again. This is the way it works through E-7, after that, I have no clue. NOW, the other way, is going to a service school. I was using Engineman / Machinery Technician. You can do this out of basic, or from the Field and go to ENA School, or whatever it is these days, MT I think. You pass and Graduate as an E-3 Designated. Top of your class, you get E-4. After that, you are back in the field and have to take the service wide exams. With the Army, I don't know how they base it, but S/Sgt seems to be rather common. It maybe that the S/Sgt is in an MOS with some crusty old SFC tying up the billet and refusing to retire.

This is a pointless question, because there could be a dozen reasons why this Soldier has been in the Army for 18 years and is still a Staff Sergeant. Limited upward progress due to MOS limitations, a desire to keep out of the Political realm present for E7 or above, or he lacked the education, or slots in schools for NCOES. None of which are relevant to whether he is a good NCO or even relevant to any discussion outside of his leadership and unit.
I've known several NCOs who were prevented from attending NCOES due to the length of time the NCO would be away from his duty station or due to Deployments. Notice, he also has 6 Deployment hash marks, showing a minimum of three deployments, and more than 36 months in theater.

Possibly in 19 years and soon to retire...