I bet Giants fans are whining about being robbed because those 2 calls were both bad and went in Dallas' favor and despite the final score could have changed things.
While both could have been called the other way, my thinking is this
On the no call, the DC defender did "hit" the Giant receiver in the back with his left hand but it did not push him forward, down or to the side, it didnt really change his body position. The only reason he did not catch the ball was because the Cowboy smacked the ball with his right hand
On the call against the Giants, what the defender was doing was holding the receivers right arm back. Who was it, I forget, was it Cobb? He was never able to bring his right arm around so he could attempt a 2 handed catch. The defender held the receivers arm back. granted it was not especially blatant but that is what I am assuming the ref saw. Plus there was a small tug at the jersey, but I am thinking that it was more about holding the arm back.
I think both were really close but the refs got them right