This makes complete sense — people who are capable of searching on their mobile device own a smartphone, people who own smartphones are either tech savvy or of a younger demographic (excuse my assumptions) but both of which are more likely to use Google.
Also, the two main smartphone platforms (iPhone and Android) have their default search engine set as Google.
I believe most (all?) BlackBerries default to Google also.
Depends on the carrier. Verizon took cash from Microsoft to set the default search on all their BlackBerries (and maybe other phones?) to Bing.