I guess it's just a matter of people hearing these phrases when they're very young, intuiting the context with limited information, and never seeing them written down until their own version is thoroughly stuck in their head.
It goes the other way, too - my dad mentioned that for a number of years in his youth, he thought the word "misled" was pronounced "my-zelled" - he'd seen it written down, had understood the meaning but not the derivation, and was well into his teens before someone called him out on it in conversation.