Flashcard tests - more cards when few categories selected for test

I have quite a few different flashcard categories, but I'm taking HSK5 soon so I decided to disable all the other categories, leaving only HSK1-5. To my surprise, however, there were actually more cards in the test than when I had all the other categories selected. How is this possible? I don't have any other filters active.
Ah...think I've figured it out. I think it's related to the "unlearned" limit. Maybe I have more "unlearned" cards in the other categories, so when I disable those categories, it frees up more unlearned cards to go into the pool.