There are some advantages to AT&T, if you have a signal in your area. But there are possible reasons for these numbers that you left out.
The $50 iPhone 3GS is 1/4 of the cost of an iPhone 4, and is only available on AT&T.
A strong brand association between AT&T and the iPhone, and Verizon and Android, has been built up over the last few years.
Verizon has a much stronger lineup of Android phones to sell than AT&T does, including LTE phones.
Most important, people who...