So I've noticed recently that top ranked posts sometimes disappear from the search results when I add another tag, even if that post has that tag.

For example, there are multiple page 1 results with the "female" tag if I just search "order:rank". But if I search "order:rank female", *some* of those posts remain in the page 1 results, and some of them disappear.

I don't understand why that happens. Can anybody explain?