I think what cancels out, or should cancel out, rather Necromorps and the flood are that fact that there seems to be an intelligent design controlling them. Same with the Boomers and the Spitters. They seem to have some semblance of intelligence.

I think another problem, is people are putting in the strain or various other products of whatever means is used to make zombies. Like for instance RE zombies, I don't think Lickers, Tyrants and other "mosters" from that series are considered Zombers per se. They are generally mutated or frankenstein'd out of a test tube. Same with the "special" infected from L4. There isn't any (as far as I know) real backstory about how the various strains of infected came to be.

As far as the Necromorphs and The flood are concerned. I think they don't count as Zombies because they are a parasitic organism that uses the hosts body to make more of its kind. Sort of as a way of reproducing. Whereas most Zombie mythology is just worried about feeding, with the spreading of the disease as being a byproduct. Otherwise we could include other monsters such as, The Thing, from the movie, "The Thing".