It's coming from the User table which is linked to the Submission table. Makes my head spin trying to keep it all straight!
Here's a link to the base, just test data in there right now.
Did you get this figured out? I looked at your base and it looks like your summary table is duplicating your Submissions table, rather than summarizing it. What, specifically, are you trying to summarize? The submissions by User by Challenge?
I think you probably need a junction table of User & Challenge, which you kind of already have.