I expect Paul did not have easy access to chord books, so had to learn from other players. Guitar chords are often a compromise as you have to fit four fingers (sometimes plus a sneaky thumb) onto six strings. I don't think Paul ever learnt much theory, but he has managed to write so much beautiful music.

Music theory is not a set of rules on how to make music, but it can explain why it sounds good and possibly give you short-cuts. Not knowing the theory may mean you find new sounds. Listening is what really matters.