Analytics doesn't seem to be logging mobile traffic from every mobile device. Certain makes and models aren't being recognized.
One of the ongoing issues with Mobile web is that of analytics data. First, there are more 'entities' involved in the actual delivery of a mobile site to a handset, than with a desktop site. Mobile carriers, a mobile device's operating system, and that actual hardware platform of the handset all play a role.
Carriers are the ultimate provider of the information for the actual 'call to see the mobile site'. If that carrier allows us to see the the full transaction - then we can report that invoformaton to the analytics tracker. However, some carriers may hide information about the device and/or which carrier they are. If this information is hidden or simply not reported, than we're missing the needed data to pass to the tracker.
However, some specific makes and models of mobile devices themselves don't share or transmit this information. As with carriers, the lack of incoming data prevents us from being able to pass analytics data to the tracker.