Zombies may be slow, but their heads are moving targets--more so than the living thanks to their tendency to shuffle--and unless you're either a hardened veteran or a sociopath, you're going to have trouble focusing. Plus they're coming at you from all sides you're you're constantly going to have to be looking around, picking the most immediate threats. It's a fair bet they won't all be lined up in a tight little firing arc for you. That all shaves seconds off your reaction time and the more of them there are and the longer it goes on for the more those seconds will mount up.
As for the heads all being on the same plane; yeah, in small groups at range you're going to get little variance, but the more there are and the closer they get, the further you'll have to shift your aim vertically.