Second minor point: why does Religion never show up as a topic in Piece of Cake? There are probably a couple of other topics that don't as well, but Religion sticks out in my mind.
Religion, Video Games and Brain Teasers don't show up in Cake or Smartest (I think there's another category missing as well).
The reason for Brain Teasers missing is quite simple - most of the questions are fill in the blank, and those that aren't are dependent on previous questions, involve a lot of thinking and don't work well in a timed game where you want to answer a question every 3 or 4 seconds (or even faster in some cases).
Video Games probably doesn't show up because the questions tend to be quite specific (and since only fans of that video game will take a quiz on it, the %ages correct will be higher than they maybe would be otherwise).
As for Religion, I'm not sure.