Another end zone rationale question

[voiceoflg]

I saw both situations in a game. The correct ruling was administered, but I'm curious as to the "why."

62 yard TD run and the ball carriers launches himself and does a mid-air somersault into the end zone. When he comes down, the ball becomes dislodged after hitting the ground. 15 yard USC penalty, but the TD stands.

Later that game a 20 yard pass into the end zone. Same player that made the earlier TD dives for the ball, grabs it, pulls it into his body, then hits the ground where the ball becomes dislodged. Incomplete pass. He had as much control of the ball after the catch as he did when somersaulting into the end zone on the run.

The officials called it right by the rule book, AFAIK, but I'm curious why the running play is a TD but the pass play is an incomplete pass. What's the reason behind those rules?