Yes, it a little confusing, and I don’t have the answer. I think it might have something to do with the actual locations. When you select “Taxonomy Term” the field groups will appear with editing a Term in the Taxonomy. When selecting “Post Taxonomy” This is more of a generic “Post Category” or “Post Tag”. I don’t think there is a really good generic way to say “Post Term in any Built in or Custom Taxonomy”. At least that’s my guess at the developer’s thinking when he created these.
