The thing with high school guys is you don't know if they will hit the ceiling that their tools portend. The thing with college guys is you don't know if what you are seeing is their ceiling. A lot can happen for a guy between his late-teens and early-20s in terms of physical development, but sometimes what you see is what you get. Again, not a crap shoot, but teams have to look at levels of competition, stage of physical development, and even attitude plays into it. It's a marginally exact science due to the wide variation of inputs.
In Shewmake's case, the Braves thought he would either fill out more and develop some power to go with further physical development or they thought he would hit enough to play SS. He doesn't appear to have gotten any bigger and his power numbers still aren't there, but if he shows he can play reasonable SS and hit at least some, he can still probably carve out a bit of a career.