More categories for Amazon listed book?

So I've had my series up on Amazon for about a year now. For the first and second book, they are only in two categories - the one's I designated. Well, actually, one I designated but the other doesn't show up, the "kindle books" one does. They are both only listed as Erotica.

But the third book has several categories. Romance, Gay Fiction, Etc, Etc and I have no clue how they got those categories!

So how do you get more categories listed if the max is 2?

Ah, yeah. It's tough to figure out how and why Amazon does anything.

But the other day I found a blog post by a fellow self publisher - I'll post the link when I get home from work - and they explain how keywords are a bit more important than tags.

So I changed all my keywords and tag and categories to romance instead of erotica (which the series is more romance) and now they are showing up under 5-6 different categories.
I'm not sure what prompted the changed, though.