I have noticed in the last couple of days that the total number of questions is not very steady. At one point there were 942k questions, then 920k, then after some time they came back to 942k, and today it shows 930k. Is this a bug?
For some of the sorts, certain questions are excluded; for example, anything voted -4 or under is not part of the "hot" tab and will not show in that question count:
I'm fairly sure this is what you're referring to.
I would imagine the question count is cached, so the servers don't have to
select count(*) from Posts where PostTypeID = 1 on the tables every time a user views a page with the view count. Or they have updated the software in a way that affected the count somehow.