Scrolling down those screenshots, I couldn't help but notice how many sites have big, square advertising banners (even if they're of the internal, 'Sign Up Now!

Scrolling down those screenshots, I couldn't help but notice how many sites have big, square advertising banners (even if they're of the internal, 'Sign Up Now!' kind) on the mid-to-lower right hand side. Is this based around some kind of science/statistical analysis?