On the main page, I have currently 16 items in the review queue according to the header, but when I click it, all the review queues are empty.
Are these skipped reviews or where does this number come from?
We already discussed that in chat a few days ago. While having no definite answer, our "guessed conclusions" are:
No idea what should have led to the current situation. While some of us observed something similar in the past (most obvious: if you suggest an edit which has to be approved, this was shown as "pending" though you certainly couldn't approve it yourself), it currently seems to be at a peak. I permanently have 20+ items shown in the "Collider™" (top toolbar), and when clicking it usually get an (almost) empty queue shown.
This is a SE-wide thingy. I see the very same on SoftRecs. So if you're interested in a definite answer/solution, your question should probably go to Meta-SE. Or just pick the relevant question out there:
This indicator is shown only to moderators and users who've gained access to the moderator tools
you're being given a birds-eye view of the actual state of review.
So this confirms what we've already guessed: it is the number of overall items in queue. And it obviously is not seen as a bug, but as a feature …