Failure Entities exist of type after removing split

Let me preface by saying: config export/import has always confused me and I don't completely understand how config & config split work. So this is a dumb question, go easy on me:)

After removing a split, I'm getting

There were errors validating the config synchronization.
Entities exist of type...

on import to another instance of the same site.

Since the config is simply being "moved" back into the main sync, why is it trying to delete config?

Thanks.