I've been trying to focus on a new set of kanji, so I put them in a custom list to use on my iPad. It seemed to work fine at first, but now I've changed the contents of the list, and whenever I start studying just the new list, it reviews other kanji as well.

Did you remove some kanji from the list, and it's still showing you those old kanji? If that's the case, it's possible that the list won't realize that you've removed those kanji yet, until it goes through the list again. You could set yourself to the beginning of the list and add through it again to make sure that the list was updated properly.