I'm having a peculiar problem. I have posts grouped into several categories, some of which are divided into subcategories.
If I look at the categories assigned to each post, they appear to be correct. If I look at the Posts-->Categories menu, the number of posts under each article is correct.
However, certain categories are being lumped together any time I try to filter by category in the admin Posts list, if you click on a category from the front end, and also on the HTML sitemap (which generates a list of posts grouped by category).
Here's a specific example: Many posts are in the parent category Model Histories, which has several subcategories, including Family Cars and Luxury Cars. There are 24 posts in Family Cars and 40 in Luxury Cars. If I go to the All Posts screen, click Filter by Category, and select "Luxury Cars," I get 64 results: all the Luxury Cars posts AND all the Family Cars posts. The same happens in the sitemap or if someone clicks either of those categories from the front end.
I've never seen this before and I don't have a clue what might be causing it or how to fix it. Does anyone have any suggestions? I'm baffled.