Was the Chargers robbed?

It was the 4th Quarter, San Diego up 38-31, with a little over a minute left to play the Denver Broncos had the ball on the Chargers' 17 yard line..Cutler rolled right and lifted his arm to pass, what happened next is the topic of discussion amongst everybody watching the game this pass Sunday. The Chargers, challenged the play, but came up with an explanation that made them sick. Cutler rolled right and was ready to pass, but before he could even extend his arm back, the ball popped out of his hands and fell somewhere around the Chargers' 10 yrd line and San Diego recovered it.

That was the situation, here's the results:
For it to be a fumble - the QB must have his arm back and the ball is knocked out, or in this case, popped/slipped out of their hands before a passing emotion. We've all saw the replays from all angles and yes, it was a fumble. However, when the ball landed on the field, one of the referees (Hochuli) blew his whistle and just like that, the play is now dead. The play cannot be challenged and the call stands.

My knowledge of football isn't as deep as some other NFL fans, but if a QB loses a ball like this and the whistle is blown then the play is basically over, done for. Well, my questions is, why isn't it the same if this happens to a receiver or a running back??

But in conclusion, Denver had possession after the ruling, scored a touchdown and ended the game with a 2-point conversion. San Diego did try to come back but the pass was deflected in the end zone with no more time on the clock. The second game in the season where the Chargers fell short of a play.