I don't believe there's some special, extra woman vs. woman "catty" thing.
Dudes are just as merciless, backstabbing, back-biting, competeitive, jealousy-prone, etc. as women.
An example:
Male co-workers and I have talked brutal s*** about dumbass, clueless male co-workers. Brutal s***.
But once, I heard this female at work talking to another female about a third female, going off about what a "waste of flesh" the third woman was.
When I heard that, I was sort of shocked and thought, "OMG! What a CATTY B****!

What an elitest, snobby, inhuman, ice-cold, heartless c***!

"
But she didn't really say anything worse than me an' the guys would say about some male loser.
So, I suspect it may have something to do with the cultural ideal...females are nurturing, sweet, considerate, sensitive, polite, sensible.....and seeing them act Human stands out more than with a man, 'cause Men are supposed to be agressive, thick-headed, callous, blunt, competitive, assertive, insensitive, emotionally under-developed except when it comes to anger, etc.