How can someone retire as an E5 or E6?

It's possible some of them were medically retired for injuries sustained in the line of duty.

If you are in the Army National Guard or Reserve your whole military career it is possible. Sometimes in Combat Arms Units moving up beyond E5 or E6 can be very hard, as people promote into the E7 or E8 positions and never leave till mandatory separation. so it is possible to only make E5 or E6 before your 20yrs.

Variety of factors. There is medical retirement. There is also the possibility that a given member could have been among those offered early retirement at 15 years. There is also the fact that some just don't want to go higher. Duty station I had in Germany we had an E-5 by the name of Bell and had been in for 18 years. Of course this was back in the late 80's early 90's. Also There is the whole deal of being prior service and coming over to another branch of the military. There is also the cases, back before policy change, where a SM got busted down and made it back but only to a certain degree.

There is a lot that can and does happen. Also, have to look at when the person was in. Not all of those you see with R next to their name are modern day. Some have been out for a few decades.

E6 can retire at 20 years. E5 most likely like everyone else has said medically retired.

I was E-6 and happily retired because one could back then(2003), didn’t want to be an E-7

I employed a 20 year E6 retired, his dd214 was accurate and his military specialty was managing events as well as military clubs on base in Europe, he said he had a "blast" for 20 years, he turned out to be a very good 2nd level supervisor, I had never come across a 20 year E6 , most 20 year people were E7 to E9

I'm curious what your AFSC was that you could have made E5 in under 4 years. Unless TIG/TIS rules have changed from when I enlisted a long damn time ago, you can't even pin E4 until 36 months TIS (30 if BTZ) plus six months TIG before you are eligible for SSgt. Add to that, waiting at least six months to test and most likely another year to actually pin the rank (due to waiting on your line number to increment during the month....

Add to that, as a 205x0 (1N5X1), the first two years I tested it was mathematically impossible to be promoted (needed 118 on SKT/PFE first year, 108 on SKT/PFE second year) in my AFSC. Theoretically I could have made it on the third attempt if I had scored 96+ on each. Cut off scores were just that high back then (AF average to E5 in the early/mid 90's was 7.5 years, 8.5 for my AFSC). Similar scores were needed when I was testing for TSgt. We lost a large number of talented Airmen simply because they hit HYT before they could break the cut off barrier. By the time I retired in 2010, you could make SSgt by just showing up to work on time 3 days out of 8.

It's common in the Navy LTC (Join to see). I worked for Navy a s a civilian for years and Ive seen many retire as an E6. Much has to do with rates and tests. Ive also seen many make E9 in 20 years.

Well sir, I have been in for over 20 years, mostly in the guard. I have seen several E4, E5, and E6 Soldiers with 20 years in (in the National Guard). Most of them were either flagged for some reason and unable to promote (but still Soldiers that their Commanders kept around for whatever reason), or they legitimately had no slots of higher grade for their particular MOS and the Soldier was unwilling/unable to move to another unit. Happens quite often in the guard actually. But I will say that the retention boards in the guard are getting more strict it seems these days so there seems to be much less of that as time goes on.

In fact, When I first joined in the 90's (guard first before going active), I met an E4 and an E6 that had both joined at the end of Vietnam, and then stayed in the guard almost the entire time (I think one of them had a small break in service though).

I retired as an E6 Tech sgt with 20 years. When you work with females as in a medical field and you have nurses who are single and have the macho effect along with their favorite boy toy and they want you out of the way for him to take your job, your backed by the hospital commander who tell them to leave you alone they get even with decreasing your evaluation. So you take your 20 and retire as I did back in 86. Politics does play a roll in the military.