Those visitors probably didn't make it to your server, and got lost during redirects (canceled the request). This makes sense if they are using 3G. As there are at least 1 (sometimes 2) redirects between the click and your site. They track the visitor before they get redirected obviously.

I have dev'd for an advertising company, have worked with several campaigning networks like HasOffers, and have found similar results. This is more than common.

Still very interesting that the bulk seems to be Android (mobile traffic). A must know if you are not targeting mobile..