a) Celtics have a worse record than the Pacers and the Bulls, two teams which over the course of the season have defended better. That gives the edge to Hibbert and Noah. The Knicks also have a better record and Tyson Chandler is still transforming what would otherwise be a terrible defensive team into a quite good one, on a nightly basis.
b) Big men have a more noticeable effect on the box score.
c) Bradley has a truly great defender as a teammate, who happens to be a big man and the anchor of the C's defense -- KG -- who takes the focus of outsiders away from his abilities.
d) Avery missed nearly half the season.
e) In any case, the smart money (i.e. people who do bball stats for a living) is on Larry Sanders as defender making the biggest impact in the league right now.
Even if we're just talking about the time since Bradley returned, I would put Sanders, Noah, and Chandler ahead of Bradley for the award. I'd probably recognize KG ahead of Bradley, too.
Avery is definitely the best defender at guard in the league, though, in my opinion.