I don't have stats to give you a firm answer, but if I were going to research this subject, I'd consider the fact that Android devices dominate developing countries. Advertisers in those countries probably can't afford large marketing budgets so their value per view may be significantly lower. Additionally, I'd argue that they probably have less free time and less data/slower speeds making them less likely to spend time grinding out Google searches like Americans, Brits, etc.

Again, not stating facts, just hypothesis.