Williams is a classic juco guy. Looks great on the highlight tape, but doesn't necessarily do the little things, especially defense, rebounding/boxing out, moving the ball, running sets, etc. that make him valuable to a winning team. His pick and roll defense and recovery defense are both well below average for a D-1 player.
I would bet $ that Jalen Hill had a far better +/- (or team impact) than Williams last year despite scoring less and shooting similar percentages.
Edit: Went back to +/- thread - He had the worst +/- on the team by far --
Harmon 100
Reaves 99
Manek 84
Hill 72
Doolittle 45
Iwuakor 38
Bieniemy 27
Kuath -1
Williams -26