NFL Network did a series on this where they followed some guys going through it. It's guys who didn't make it or barely played and i think they limit it to like 3 years out of college.
Every year, there are a few guys who come out of it. You can find guys who maybe weren't in great shape coming out of college and with a year off, they did nothing but workout, so there are guys who may have improved quite a bit physically since they were originally scouted.
Chris Hogan WR with Buffalo came through this process. He played lacrosse at Penn State, then he transferred to a small school (Monmouth i think) and played 1 year of football. Because of that he was off the radar. i believe he got into cross fit and won some of those competitions so then he decided to give the regional combine a try. I think he ran a sub 4.4 and had other impressive workout numbers so he got his shot. He's becoming a good #3 WR for Buffalo.
That's pretty much what you can find in these combines. Guys from small schools who may not have been fully scouted and guys from big schools who may have matured physically or mentally.