It's quite likely that that's where I got it and then just forgot.

This happens to me alot. Motivated by curiosity I will make the effort to acquire a piece of knowledge but since I don't have a functional use for such knowledge parts of it slip out of memory..

..then some time after I will have a functional need for the knowledge I learnt previously but relearning something that is already somewhat familiar isn't that captivating so it ends up being a struggle the second time.