Generally when you ask people how attractive they think they are on a scale of 1-10 they're more apt to rate themselves an "average" 5 than less or more honestly. At least what I've heard, I'll need to double-back and look for that study.

Therefore I feel that people could be looking at the forum sections and seeing advanced as too much over their 'game' and free being beneath them. So Casual feels like the acceptable middle-ground between both. This is compounded by Casual being the busier forum which ends up feeding itself with more activity.

Free RP is inhabited by the crowd simply too intimidated probably because they could be young or more casual than casual, and Advanced the handful of people that feel or have been convinced they're better than the rest.