In order for the spent cartridge to be expelled, the pistol's slide has to cycle. This cycling could have been prevented by:
1) Somebody's hand holding the slide, which would support the argument that there was a struggle for the gun, or
2) The back of the slide being in contact with something that prevented its rearward motion (for example, if the back of the gun's slide had been in contact with Zimmerman's body)
The first scenario above seems more likely to me.
Another question:
Would it be possible to shot someone with the gun still in the holster?