If I have understood correctly, I think it is related to this:
For example, today I read through my unreads here on Meta. Then I read through new topics in a couple of categories I’m watching. Having read the last of these in Features, I’m presented with this:
However, visiting the “New” page shows there are still at least two new threads in UX, a category I’m watching, but these are not shown to me as suggested topics. Instead, I see Support and Dev, which I’m not watching.
As I said in the other thread, my expectation would be that “Suggested” prioritises categories I’m watching, but this is not the case for new topics.