Maybe he was just repeating himself?
Pure speculation: I can envision a scene with GZ stopping pursuit, bending over to catch his breath, and then repeating himself to the dispatcher. It's getting dark, or is already dark out, it's rainy, visibility is poor, and GZ doesn't realize how close TM is to him. After GZ gets off the phone, TM confronts him (or GZ realizes TM is nearby and confronts him). Speculation only.
The biggest problem that I have with this case is that, unfortunately, GZ's story is plausible. And the scant witness account(s) and evidence appear to support his version (or most of his version) of events. JMO, OMO, MOO, etc.
If it turns out GZ's story is correct then what? would JJ and AS still be calling for the arrest of Zimmerman?