I know many will disagree with me, but I've always had the feeling that advertising on mobile apps is there to exploit user distraction. I haven't got any source to back my theory, nor any conversion rate stats, but I would say that a big chunk of clicks come from user mistakes, rather than genuine interest in the ads, and the same would apply to desktop applications imo.

This might happen on the web too, but with a lower order of magnitude, since the average internet surfer is more likely ready to evaluate an offer/purchase.