This doesn't compute.

I'm trying to put a door sweep on my door that goes to the garage and the
crack between it and the floor is not uniform. It appears when the the door
is closed that the gap is about 1/2" by the hinges and about 1/4" by the
door knob. So I toss a level on the floor and see that it is indeed level.
So I put a level on the top of the door, and it is level. If the door, the
floor are both level then why is the crack wider on one end?