T-mobile has been actually retrofitting their 3g towers with 1900MHz frequency support in some areas in the last few months (including NY), which lets unlocked iPhone users actually use 3G on their network.

EDIT: Drachen explained it already.

Also, be aware T-Mobile uses LTE band 4, which is only supported by the AT&T version of the iPhone 5 (A1428). Both the unlocked version and the verizon versions do not support that band.