I believe this happens because the "ratings czar" doesn't pay any attention to the player until he is added to a team. Once a player is signed to an active roster the "ratings czar" will then adjust the ratings so that he fits in with the team.
If a 78 OVR Linebacker is signed to a team where the average for Linebackers is 75 OVR, unless the free agent is a star player the "ratings czar" will lower ratings to artificially make him look like a 2nd team LB on that roster. It's sad.