Sherman is the best defensive player and Earl was 5th with Watt, Quinn and Kuechly in the middle. I wouldn't sweat the actual place too much because the poll wasn't set up properly - it just says name the 20 best players regardless of position - offensive guys are always going to have more recognition.
Think of it that Earl is a top-5 defensive player in the league and Sherman is the best. I can live with that. When you compare FS to QBs - of course the QBs are going to be rated more highly.
I think the real perplexing thing is how Tom Brady is still a top-10 player and Aaron Rodgers isn't.