I think your missing out on an opportunity Jel, at Coolmobilegroup we rotate a bunch of different offers based on the geo/carrier/isp and find the one's that convert best for it.
German/Android/wifi is very different than say Spanish/Android/xfera moviles s.a.
(Which were getting 20.73 Euros per eCPM today just wanted to plug that
)
Not only do the languages and creative have to be different but options like App Installs and auto download offers can be made available to different types of traffic. Going with just the sites niche offer type you'd be losing out.