People have their own preferences regarding the sort of pictures they like to make, and the sort of figures they use in those pictures; and maybe those preferences also inform their choice of forum on some level. Once a forum starts to show a trend towards a certain kind of art, then it will be self perpetuating - people will tend to choose that forum if they like that style. I hadn't really noticed it myself, but it also depends on what part of those forums you visit. The galleries may well have a different bias to the advice forums; members often stick to one or the other.