[Plugin: WP-PageNavi] Possible bug?

I have a custom template set up so that 9 posts show up on each page. However, in one particular category, there are 10 posts. So the category paginates as it should – cuts off at post #9 and provides a “Next” link to Page 2 to see the 10th post.

However, when I click “Next” I get a 404 page instead of the 10th post.

here’s the kicker – if I add a new post in that category (and make it 11 posts instead of 10) pagination works fine, and you see posts 10 and 11 on page2.

??? Why would it do that? If only one post shows up on the last page, it goes to 404, but if more than one is there, it’s fine?