It's normal. Usually despite being on the same network, the updates for the devices are not released at the same time due to product codes and such. Also, it could be that T-Mobile released the update via OTA method, in which case you need to be using a T-Mobile SIM if I remember correctly. Its a bit more difficult with HTC as I don't know if they have a PC update program. With a Samsung you can just install Kies on a PC or Mac and update your phone where ever you are in the world or whatever carrier you are currently using your phone on. If HTC has a similar program you may have to use that.