Google released Android 5.1 in a stock form last week, and now the carriers are starting to issue updates with devices that have their SIM cards inside. Sprint and Verizon have released updates so far, and now T-Mobile is joining the club.
This specific build of Android 5.1 is LYM47M, which is unique from the other updates. The radio version is still 01.02.95, instead of the more updated version 01.02.98, which is on Verizon. There’s no reason why the T-Mobile radio version is behind, but it will probably be updated after the device gets Wi-Fi calling, which T-Mobile has been aiming for.
So if you have a Nexus 6 with a T-Mobile SIM card, you can now download Android 5.1 OTA. Hopefully AT&T and smaller carriers’ updates arrive soon.