LEFT JOIN: Am I missing something?

Summary

Why isn't the LEFT JOIN giving me all of the "LEFT" records?

Content

I started with LearnCenters, Category and Category Mappings and got a distinct set of all the items I'm looking for. Then I attempted to LEFT JOIN User Progress to see which users had completed which items. It's only returning records having User Progress. I'm not getting any NULL items from Category Mappings where there's not a corresponding User Progress record. I'm very accustomed to writing SQL, so I'm not sure what I'm doing wrong here, but it's not functioning as a left join. I should receive