It's way more swingy on a low cr monster designed to fight low level people.

And the point is that cr itself doesn't measure that kind of ability well, so you have to take precautions based on play testing. Not that it shouldn't happen, but that any strict number based thing has a wide variance.

And until it shrieks, how do they know it's special? If it's obviously different, that's one thing. But monsters don't come with name plates, and I'd imagine zombies are a pretty diverse bunch already.