Now, I've got no idea how Mark plans to scale the scores across three categories and take into account each game's amount of reviews. But this is what makes the most sense to me:
Gooey Castle is the top game in Design, so I'll use it as an example.
Design: 4.890 Theme: 4.890 Originality: 4.918
Add them all together = 14.698
Divide by three (there's three categories) = 4.899
Divide by amount of ratings (in this case 73) = 0.067
Subtract 5 (max rating) and convert to a positive for your scaled average score = 4.932
So yeah, this may or may not be how the top 100 are chosen. I've got no idea, but this is how I would do it.