From SimGuruQARobo:
If I'm understanding the topic correctly, I believe this is by design. The pack filters are intended to be inclusive rather than exclusive. In other words, they will display any gallery items that include those packs, regardless of whether other, unselected packs are also included in the item.
I don't really know how to put it in other words. It means exactly what it says; if you have a pack checked it'll show lots with the pack. Last time I checked, it also shows lots that don't have the pack. So in a practical sense, the answer to your question "How do the gallery filters work?" is "They don't."
