For all the arguments about Mac and Zappe (and they both very well could just suck), I've noticed the playcalling is just way different for each guy. Whenever Zappe is in there, it's a ton of screens, checkdowns, occasional slants. But he almost never attempts a deep ball. Whereas Mac does and they typically get intercepted.
So while it "looks" better for Zappe and he might not have many turnovers to his name, it's because the playcalling is extremely simple and not as risky as when Mac's in there.
It was the same last year too btw. Play action, screens. But when Mac was there it was a lot of deep shots and "50/50 balls" to our bum receivers which predictably didn't end well.
I just think if Zappe had to deal with the same sort of pressure and was told to throw it downfield more, he'd have a lot more pedestrian, if not horrible stat lines too with turnovers.