Problem solved! For future reference, I tried something that seems to have worked:
Since I requested 9 products per page and it only showed 8, I increased the number of products from 9 to 10, thinking it would show 9. Imagine my surprise when it showed 10!
I then reduced the number from 10 to 9–and, lo and behold, it now shows 9.
The lesson is that we have to play with this a bit until it does what we asked it to.