I'm on Samsung's website comparing the antennas between the two versions.
Att version: 2G GSM: GSM850, GSM900, DCS1800, PCS1900;
3G UMTS: B1(2100), B2(1900), B5(850);
4G FDD LTE: B1(2100), B2(1900), B3(1800), B4(AWS), B5(850), B7(2600), B17(700), B20(800);
T-Mobile version: 2G GSM: GSM850, GSM900, DCS1800, PCS1900;
3G UMTS: B1(2100), B2(1900), B4(AWS), B5(850);
4G FDD LTE: B1(2100), B2(1900), B3(1800), B4(AWS), B5(850), B7(2600), B8(900), B12(700), B17(700);
Maybe the differences have something to do with the data speeds you're getting.