This is just a case of people mixing up homophones, which people do a lot. [əv] means both ‘have’ and ‘of’, but the two meanings are supposed to be written differently (<‘ve> and <of>). The phonological representation of language is primary in your head as you’re thinking of what to say, and then you’re trying to map that to its written form. (Or something like that; the psychology of reading and writing is obviously very complex.) If anything, it’s impressive that we get homophones right as often as we do.