It might be because I often delete cards in the middle of review sections (e.g., if I suspect a word isn't commonly used enough to be worth periodically reviewing). Even after I delete them, blank placeholders for those cards remain in that review cycle, and I have to assign them a score to move on. The only way to get them to stop showing up completely without restarting the review session is to assign them a positive score, which might change counts in the statistics based on cards that no longer exist.
By the way, the reason I don't exclude them from all sessions is because I really want to get them out of my database in case I share my flashcards with others in the future (I don't want it to be full of cards that aren't useful).