The difference between an "incompletion" and a "completion followed by a fumble" is in the rules for a completion.
A completion has to meet three criteria:
- The receiver needs to gain control of the ball.
- The receiver needs to have both feet or any other body part (except hands) on the ground inbounds.
- The receiver needs to maintain control of the ball long enough to make a "football move."
If any of these three criteria are not met, it is an incompletion. If all three are met, it is a completion, and if the ball is dropped, a fumble.
From the 2013 Official Playing Rules of the NFL, 8.1.3:
COMPLETED OR INTERCEPTED PASS
Article 3 Completed or Intercepted Pass. A player who makes a catch may advance the ball. A forward pass is complete
(by the offense) or intercepted (by the defense) if a player, who is inbounds:
(a) secures control of the ball in his hands or arms prior to the ball touching the ground; and
(b) touches the ground inbounds with both feet or with any part of his body other than his hands; and
(c) maintains control of the ball long enough, after (a) and (b) have been fulfilled, to enable him to perform any act
common to the game (i.e., maintaining control long enough to pitch it, pass it, advance with it, or avoid or ward off an
Note 1: It is not necessary that he commit such an act, provided that he maintains control of the ball long enough to do so.
Note 2: If a player has control of the ball, a slight movement of the ball will not be considered a loss of possession. He must
lose control of the ball in order to rule that there has been a loss of possession.