I think the advertises picked randomized. The more products for a character are in store, the greater the chance to appear in the advertises. Maybe, there are a few more values addressed. This morning (after reading this thread), I looked the first time explicit at the at the Top and saw a product for Genesis 8, one for LA Feme and one Scenery product for both Studio and Poser.

But you are right, the most of the time there are genesis products only! seldom a poser only product.