No surprise, really...they're presumably safest for the same reason that they're the least chosen. Similarly, if they had broken down the OL by position, guards and centers would have proven far safer than left tackles.
There's an element of self-fulfilling prophesy here. S, G & C are traditionally the lower-paid positions populated by good, smart football players who are not elite athletes. For a player at a traditionally low-rent position to crack the 1st round he's probably an exceptional prospect with polished skills, while teams will roll the dice on a guy with measurables like JaMarcus Russell Ted Ginn at the glamour positions.