In spoken language, ‘would’ve’ and ‘would of’ are homophones so for all intents and purposes it doesn’t matter which one the person thinks they are saying. But they need to know which is which when they write.
But folks mostly don’t write. That’s part of what’s going on with a lot of this stuff. People talk all the time, but write rarely (unless they hang out on internet boards), then they’re cranking out an e-mail and they realize they don’t actually know the real words behind half the crap they say, especially when you’re a fan of contracts. When I say or write “I’m” I don’t think “I am” and shrink it, I think “I’m” in my brain (and the spell checker) “I’m” is a word. And it could be a word I’ve used so regularly and consistently I don’t even know what the parts are. Nobody says “would have”, between the contraction and the fact that it’s part of a sentence structure that people don’t use often it’s not something on the everyday tongue. So when they do try to use it they get it wrong. Of course if they do it enough it stops being wrong. That’s how language works.