Does anyone have an explanation on why the 3G implemented in the US is different from the one that is implemented from Asia?

From what I know:

US: There is Uplink and Downlink and both are on different Frequency
Ex - AT&T 850MHz uplink and 1900MHz downlink
Ex - T-Mobile 1700MHz uplink 2100MHz downlink

Asia: Both Uplink and Downlink is on the same frequency
Ex - Singtel 2100MHz

Reference: http://www.breakitdownblog.com/3g-iphone-may-not-work-on-t-mobile-3g-network/