Knowing what you're doing doesn't mean you're omniscient. In an unbounded domain it might mean you have very good intuition for what to do and you're often directionally correct. There's still a lot of room for learning there.
Absolutely. There is a massive spectrum between knowing what you are doing and knowing everything there is to know about a subject. We are all constantly adding to our knowledge base… no matter our starting point.
In addition to the base knowledge is the ability to do something well. I know how to do a lot of home maintenance tasks. Due to a lack of practice, I don’t have the skill to do them all very well.