I'm getting players 100+ points overall apart receiving the same overall letter grade rating at Levels 1-3. My gut is telling me there's some sort of bug here, but since I'm not 100 percent of the intent for the "different rankings by position" change that was in effect for Season 3 beta, there's a chance this is just an intended difference stemming from that (but I've got people at the same position getting the same eval grad of B despite 110 rating points in difference, so I certainly hope things aren't functioning as intended...)
Rather than be redundant, see the "Seble -- Letter grades by position/role" topic for a further list