Bush is more agile and gets up to top-end a bit quicker, but he is lacking the SPD and Jumping ability compared to Shields. I have Bush 6 points higher in AWR, mostly due to his greater experience in the league, and 9 points higher in play recognition for the same reason.
1. All the scouting data you see is PRELIMINARY and is not finalized. Read the FBGratings thread for more detail.
2. The scouting data is as of the end of last season, as well as the OVR ratings. That is to say, directly after the SB was played.
3. Shields had been upwards of a 71 before the SB was played.
4. Keep in mind that this system of ratings is designed to stretch out the ratings. Your "average" player that is a 75 in Madden according to EAs ratings is more like a 70 in mine.
5. Experience and injuries affect the OVR rating significantly, and they are often overcome by physical skills in the ratings system.
The point of the nose picking statement was to show how asinine it would be to not consider Bush's efforts that day in coverage.