How does an E6 have 6 service stripes?

Prior service. Break in service. Maybe he had an injury that took some time to recover from. There's always a reason. Or maybe he had some discipline issues. Or maybe he's wearing stripes he hasn't earned. Who knows?

I have 5 (15 years) and still came out as SSG... but I had a 16 plus years break in service. Served from 1968-1974 and 1990-1999. I had to drop a pay grade when I got back in. (Didn't have to go through Basic though. Just trained for my new MOS's)
P.S. I know this post started TWO YEARS AGO, but I just thought I'd respond since others have also responded fairly recently.

Sgt Christopher Wenzel SGT Sean O'Hara SFC Jordan Gaudard GySgt John Olson Sgt (Join to see) CW3 (Join to see) SSG Owen Kingsley SGT Carl Blas SSG Jason Penn Capt (Join to see) CW3 Kevin Storm CPL(P) (Join to see) SPC Thomas Baldwin SFC Jim Ruether MSG Francisco Ojeda

He might have been in the NG or Reserves before he went active duty. Or he's in a MOS that the promotions for E-7 are frozen, I was in the NG for eight years when I went back on Active status. Then I went in the Reserves I was and E-6 when I retired and had a total of 28 years service.

I had 7 service stripes as an E-6, 3 years Active, the rest National Guard. Then I was promoted to E-7, I eventually retired as an MSG with 11 service stripes. I really didn't want those many years, I was getting physically exhausted.
I was a dual status Mil-tech, that is a person who has a civilian Civil Service job for the National Guard, with a unique condition of employment of being a military member of the National Guard. So to get to be eligible to retire from Civil Service you have to have over 30 years of service. Now you have two careers running side by side, any hiccup on the military side and your outa there, you loose your "civilian" job. I was finally deemed non deployable in 2004, and retired from the military; I "should" have lost my job instantly, but because of the severity of deployments of the facility, they let me stay till March 2005, only 3 months, till I reached eligibility to retire "normally" Otherwise I would have had to apply for disability and fought and waited a year or two with no income. I certainly wasn't a good ole boy, I fought tooth and nail over stupid things they would concoct..

An E-6 can serve 20 years before he/she must retire.

My cousin was an officer and got out at around 15 yrs. Later he decided to come back in, but not as an officer, as a SSG to finish out his 20. He didn't want the responsibility of an officer, but when he retired, he retired at his highest rank, so as an SSG he ended up with 7 service stripes total.

Because he's put in the time. He may have Guard time , he my be a service transfer there could be a number of reasons. I personally will have 6 service stripes in SEP . But at the same time my time in grade change 3 times because I went from Active duty Army to Air National Guard to Active Duty Air Force back to active duty Army. I have nothing derogatory in my records have all good NCO reviews from Army and Air Force and school qualified in 4 MOS's and have completed SLC in 2012 so I say all this to say just because he is and NCO with that amount of service stripes and his rank is that of SSG doesn't make him below standard promotion at that level is all about what the board is looking for and the vacancies in your career field

I'm at E6 who will have 17 years later on this summer. Didn't make E5 until 11 years in due to over strength MOS and being blocked by an E6 who thought his skill level was above going to ALC. Should I have done more to further my career? Absolutely. However, I was happy where I was and with what I was doing.

Hell, I know of an E-4 with at least 10 service stripes.

It's not that uncommon. Some mos' are hard to get promoted in. I'm at 15 years and I've spent 10 years of that as a Ssg. Many SSGs in my MoS have retired as SSGs at 20years