Well, Rodney's improvement was amazing in terms of ERA, innings, and many advanced stats. Not only is there the glorified save total, but even other ways of looking at his performance like shutdowns and meltdowns said he had a terrific year. Finally figuring out how to avoid walks, something that no pitching coach other than Hickey could get him to do, was huge. In terms of WAR, he went from below replacement level (-0.2) to 2.4, which is very high for a reliever. In terms of "win probability added," he basically turned a negative career into a positive in one season (2012 WPA 4.82, career 2.44).
You can argue he was a never was and really was not making a comeback, you can argue that relievers' performance bounces around so much that the notion of a comeback is ridiculous, and you can even point out that he might not have been as good as Jake McGee among Rays relievers (1.81 FIP v. 2.13). However, there's no separate comeback pitcher / reliever of the year and comeback position player / DH of the year, so I can see how he got a ton of votes.
I agree Dunn deserved it more, because he was very good, then absolutely atrocious last year, and then good again. Dunn was probably hurt by folks who penalize his average. He only went from .160 to .204. Also, his overall performance I don't think was as good as his time here, at least in terms of AVG and OPS. Hard for me to ignore a 30 HR improvement from 2011, hitting his most HRs since 2004. His WAR improvement was even more than Rodney, going from -3.0 to 1.7.