How does an E6 have 6 service stripes?

I employed thousands of veterans with a paramilitary company for 41 years, the MOS, skill sets, performance and training typically reflected NCO or Officer rank after 20 years

Probably a reservist...or Article 15, who knows.

Maybe he was in a slow promoting MOS and points were always maxed out. Maybe never got selected for SFC due to not having appropriate schools, there tons of scenarios. He could be one of the sharpest NCOs around for all you know. I met plenty that were hard chargers, just didn't get promoted for certain reasons.

I lived next door to a retired E6 so it's not unheard of, he might have had UCMJ problems after he passed the 18 year lock in.

quite easy actually - first, he probably is not in a combat arms MOS, Competition in non combat arms is very intense, and has a very slow promotion rate, It is quite possible that as an E-5 he was promotable, but did not have the points required to pin SSG, one of the ways the Army controls NCO population within MOS's is by making the points to promote 798, or in soldier speak, seven ninety never. It is within the realm of possibility that this NCO doesn't have the time in grade requirement to be looked at by a centralized board for promotion to SFC. Second, He was most likely grandfathered when the Army switched the RCP for the ranks, I see a CAB on his chest, which means he definitely was in the service when the RCP for a Sergeant was 15 years, a SGT (P) 20, and a SSG 23. I believe the services reverted back to the original RCP's in 2015, when the actions in Iraq ended, and and the drawback from Afghanistan.

Actually I had an E6 who was a former O-3 and was reactivated after 9-11 and became one of our Fire Department Fire Chiefs- more common than you think, I got out as a CPL with 10 Years in, some MOS’s don’t have a lot of slots

You did your own math. It is simple and highly normal for an SSG to have 18, even retire at. There are no reasons necessary. To make SSG is very honorable and challenging in the Army, RESPECT Private.

There are several valid possibilities. 1 – the NCO could be in a CMF that has fewer SNCOs than say, CMF11 or CMF18, 2 – the photo was taken prior to the NCO advancing to the rank of SFC. He could be on the next promotion list, 3 – he could have had a break in service and has only recently returned to Active Duty and 4 – he could have made a lateral transfer from another branch (E.G., Navy, USMC, etc.) along with reclassification into what might be a totally different CMF. And as, SSG Terry Kemp mentions, there are still a lot of NCOs who retire as SSG in both the Army and USMC, PO1 in the Navy and USCG and T/Sgt in the USAF.

Some MOS's are hard to get promoted in. I went to AIT, Intl Analyst, we had 4 students that were E-6. They all have 15+ years. Looks as though he has quite a few combat time stripes as well.

I've known a PFC who was allowed to retire as a PFC...some times Soldiers only rise to the level of their ability. Now, you will never see that, but in the past it was pretty common. I know several people who preferred to continue their service as Specialists, they didn't want the leadership responsibilities of an NCO.
I retired as an E7, I should have risen higher, but I stayed in an MOS that is a very small community, Public Affairs. There is not much upward potential when there are only 5 E8 positions, and ONE E9. There were 37 of us competing for those 5 E8 slots. However, I served as a 1SG in Iraq in 05-06...I just couldn't wear the diamond.
If I had to do it again, I would have returned to one of my other MOS, 88M, 79R or 25U...though 25U didn't have an E9.
I remained an E7, because I chose to stay in a field that I had little chance of promotion.
Don't judge a situation unless you have all the background information.