I won't defend PFF's methodology or the really stupid idea of using their numbers to compare across positions, but...
Dez simply didn't have an elite-receiver year last year. His catch percentage was too low (58%), his ypc was too low (13.3) and he fumbled too often. He was much more productive in 2012. As I've said before, it wasn't all his fault, but we're talking about his 2013 production here, not his talent or ability or ceiling.
He was great in the red zone. You can't take that away from him. But apart from that, his numbers were what they were primarily because of the sheer volume of passes thrown his way.
Someone mentioned Keenan Allen. He had 22 fewer catches than Dez, but 56 fewer passes thrown his way (his catch rate was a tremendous 68%). And his ypc was a yard and a half better, 14.73 to Dez's 13.26. Plus, he scored TDs at essentially the same rate (per pass thrown to him) as Dez. So yeah, he did have a better season. Less volume, but he did a lot more with what came his way, and a lot fewer passes ended up on the ground.
I for one hope we see the Romo/Dez combination of 2012 out there this year, not the 2013 version.