The standard for receivers is that if a pass is catchable it should be caught. Period. Wes Welker is one of the best in the game because he nearly always catches balls like the one he missed. That doesn't absolve the QB for being a little off, but in reality all passes aren't going to be perfect.
"Catchability" is not an objective phenomenon.
One could with equal justice argue that if Wes Welker - who routinely catches balls plenty of other receivers would not consider "catchable" - cannot catch it, then it wasn't actually catchable.
When passes are made and men of the caliber of Gronkowski, Welker and Hernandez can't grab them - and they were all from the same passer - then maybe we look at the passer too.