There's N channels for each radio technology: 1XRTT, 3G, EVDO-RevA and RevB, LTE, etc.
The phone gets informed by the carrier which channel it is on, and depending on the channel, it will bring up the antenna more, or less often, to receive things like SMS, PTT, that should come in a timely manner. There are many strategies to keep the traffic channel up, or to trip and dip into the network less frequently.
You also do not have any control of which traffic channel you will be on, as that's pushed down to you depending on congestion, and signal strength, etc.
They work the exact same way, so I suspect they are dipping into the traffic channel less often (as well as getting fewer updates from the network) for T-Mobile than for VzW.