This isn't quite true. Listed in the original doc are the transmit frequencies of the phone, which correspond to GSM900, GSM1800 and UMTS2100.
The uplink of UMTS 2100 is in the 1900MHz range and the uplink of GSM1800 is in the 1700MHz range, which causes this confusion.
It's not a coincidence that China Unicom uses these three standards in its network (GSM900, GSM1800 and UMTS2100)
So don't get excited yet about a T-Mobile USA iPhone...