You asked if OP was sure they are android devices and suggested they may be spoofing user agents but it would be relatively easy to tell if they were actually mobile devices on a mobile network by the IP address. Assuming that they are mobile devices on a mobile network, the next question would be how would someone do this other than a botnet of mobile devices, which is essentially what luminati.io is, albeit a little more legitimate. My comment was really just suggested reading for anyone curious.

Quick way to get lots of IPs by crowd sourcing frameworks usually used for app testing and QA