Aha! I see now. Yeah, I wasn't reading it like that. That totally makes sense. That way you can have one person each working on Kimberly/Roy in each category as well as someone else doing mixed-bag. Really, if someone REALLY wants that pairing, it's flexible enough. I mean, if they meant to take mixed bag but someone already nabbed it, they can still work on a category or two while they wait. :)
Also, just curious...to complete the whole 50-set...is there a rule as to whether that has to use the same character/pairing/fandom or not? Could I get credit for the set by doing one category of Kimberly/Greed and one category of Mikaela/Bumblebee and one of...etc etc? The completist in me wants to root for it having to be the same pairing, but...omg that's a lot of one pairing.