Custom theme appears to be multiple themes

I've created a custom theme for use in my presentations, and saved it in Keynote's default location. I can use it successfully, but there's an annoying issue: When I use the "Masters" drop-down menu in the toolbar, the masters are listed in three different sub-categories. It's as if my theme has multiple themes within it and my master slides are partitioned among them. And I suspect that's sort of what happened since in creating the theme, I used slides from several other themes (like the default White, and an earlier theme I'd created) as starting points.

Anyone know how to fix this problem? How do I tell Keynote to collapse my master slides down to one theme?