Would it be fair to say that the first thing he saw was what looked liked strikes (when the guy on top had his back to him) and the later, most recent thing before calling the cops was more of a struggle moving from the sidewalk to the grass?
Personally, I think John not seeing strikes fits GZ's narrative better. GZ's account had TM slamming his head on the concrete and GZ doing his best to slide off of it to avoid more damage to his head. IT seems to me this is most likely the part that W6 witnessed.
I think it's possible that he witnessed actions that fit both his earlier and later descriptions to police, but he was conflicted over exactly how he should characterize the altercation. If you ever witness fights (even professional ones) where one person is on the top and another on the bottom, the action is very muddled and chaotic. It's plausible that, even within the course of 15 seconds, Martin was engaged in grappling for control of his wrists with Zimmerman, trying to sneak sporatic punches through when he had the chance, slaming zimmermans head onto the ground when his punches were blocked, and wrestling to maintain the dominant position as Zimmerman attempted to move onto the grass.
W6 could conceivably have been conflicted about emphasizing certain actions and thereby making a definitive characterization/statement regarding whether Martin was clearly the dominant aggressor in the fight who was raining down punches and headslams, or whether the segment of the altercation he saw was just a stage in the fight where Martin simply had the upper hand and was trying, albeit violently, to resist Zimmerman.