"Mobile device" does not always mean "cell phone". There's a whole batch of devices that use IP communication - things like dynamic street signs, cameras, call boxes, remote sensors. Lots of stuff you see/interact with (or at least, get data from) use the mobile network and require IP communication. So don't automatically assume that this is saying "everyone has 7 cell phones", because if they meant "cell phones", they would say "cell phones". Instead, read it as "devices that use the mobile network" -- even if the devices themselves aren't really "mobile" per se.