Could the explanation be that Windows RT users prefer to pay for apps rather than to be served -- and to click -- ads? That's certainly the case for me. I own a Windows RT tablet and spent about $10 on apps thus far, including on Book Bazaar Reader, GVoice, and IM+. When there's a way to get rid of ads by paying for an ad-free experience in apps I value, I do.
Microsoft is also encouraging more significant apps by setting the minimum price in its app store to $1.50. I can easily imagine that more significant apps are more tempting to buy outright rather than to live with ads.