That would be "signal strength", not necessarily "speed"
So, that means you're probably a handful of hops away from the central
server, which slows things down? Or there's something on your local tower
that hasn't been upgraded to support faster speeds?
Best I can liken it to is your wireless network card. If you have an "N"
card in your laptop, you only get "N" speeds if everything between your
laptop and the Internets. Wireless N speeds support 54mbps (108 in some
brand-to-brand connections). If you have a 10mbps hub in the middle, even
though you've got a fast connection at one point, something in the middle
has slowed you down, so the final speed is limited by the slowest
connection.
Again, that's me not knowing anything about T-Mobile's infrastructure, but
hypothesizing about various known network infrastructures.