Why do you keep having mixed up dreams?

That's just the mind's way of entertaining you while you sleep. The nonsensical order of dream 'parts' is like a puzzle that you can never put together... because none of them belong to the 'big picture.'

Dream fragments that you speak of can't be categorized. Dreaming takes place in semi-conscious state, therefore 'dream content' cannot be organized in a manner customary to a state of awareness (awake).