The B+F is awarded to the person who performs 'their role' the greatest. I think because someone's "role" can vary immensely between players, Dustin's "role" was to average 30+ possessions/match, kick 2 goals a match and give off a don't argue for the media as well.
Jackson's role was to not shank it and shutdown the opposition's best (which he did on Ablett quite well last year)
A lot of expectation was placed on Dusty, and being flogged in the first part of 2013 of not being able to run out 4 quarters would've hurt him. At the same time no one expected much from 'Hackson' so when he wasn't completely useless everyone was up in arms and gave him the B+F for performing his "role."
it's a real shame but I suppose they need some sort of metric to give the defenders a chance to win the B+F otherwise the Jack Dyer medal might as well be a rehashed brownlow night.