That is actually incorrect, AT&T's 3G is the same frequencies that the rest of the world uses. T-Mobiles frequencies was purchased in this country at that frequency because it was free spectrum and there was not enough of the current spectrum available. Also the reason behind googles desire to work with t-Mobile is because t-mobile introduced android first and pushed android hard in the early days so for loyalty google rewards them with first google only phones
Actually, for 3G, AT&T uses the 850/1900mhz bands, T-Mobile uses the 1700 AWS
band and the rest of the word, except for Canada and a few other cdma networks
use the 2100 mhz band for their 3g HSPA networks.