Not sure if this a bug or I'm missing something here
Unused tags are deleted, unless they're synonyms of another tag. But it does take a while for the tags to be completely removed from the system. Instead of being deleted immediately, they are silently and automatically removed after a period of time goes by with zero questions carrying that tag.
Browsing the list of tags by popularity and then looking at the last page is a fairly uncommon use case. Those tags are by definition the least popular tags on the entire website, probably not the ones that you wanted to see.
In fact, most of the tags listed on that page are synonyms of other tags. You can verify this by clicking on the tag, and then clicking the "synonyms" link at the top of the resulting page. xp, for example, is a synonym of the windows-xp tag.
For more about tag synonyms, see this question: What are tag synonyms? How do they work?
I'm not sure where you think the bug is; this seems like expected behavior to me.
We already do this, see