We hear "could of" because when spoken the h sound in the word "have" gets dropped. This happens so we can speak more quickly. also the f sound in "of" is softer than the v sound in 've. When most people speak they don't even say 'of', but say 'a' (I woulda, shoulda, coulda) this blending and dropping of sound makes speaking faster and easier. Spoken English and written English are very different. No native speaker annunciates every sound when they speak otherwise they'd sound like a pretentious dork or a robot. "Gonna" is not grammatically correct either, but it is the way we say "going to". We say "n" instead of "and", and we say "in" instead of "ing". A perfectly written English sentence: "I could have went shopping today." This is what you'd most likely hear in America: "I coulda wen shopin taday".