For this the best way to explain it is to have you go look at the Coaching tab and then formations. In there it tells you where the first OL1, OL2, OL3, etc are on the field. For offensive line, they are treated as a "unit" where the engine averages out their numbers. That is the only position that that happens, the rest are all based on the individual player's ratings. Once you're out of DIII you will get more options when it comes to formations and depth charts.