Caucasians serve a bigger market compared to Asians and there is more demand, hence a premium price tag (keeping in mind thst Aus99 is not a reflection of the entire market). I know plenty of my friends who never had experienced a session with Asian girls as their preference is very different so these websites -as you may gather from their design and language- are targeting Chinese customers who can pay these rates and prefer to converse in their language.
I don't know how busy they are and if all girls get business but I think sooner or later they will do a shift here or there in shops. The current higher rates of Chinese girls in 42G /No5 might be linked to these agencies. Personally I think premium at No5/42G is a rip off, but that's just me and I'm very picky.