The National Advertising Division asked T-Mobile to stop claiming that its network is faster than Verizon's.

Speed tests are measuring the network status at a certain moment in time. There are other variables that will determine the actual speed.

What should I expect as a T-Mobile customer?

That depends on where you are AND how much data you've used compared to the other T-Mobile customers around you. I've been averaging about 60-70gb per month, but normal usage has doubled for me since testing DirecTV Now (pretty good streaming TV option, but not without its problems with some channels at certain times...a similar problem to T-Mobile's speed consistency during peak volume).

I have 5 days remaining until my phone's data cycle renews. I have used 138gb. Today was a Friday. The airport was busy (meaning there were lots of mobile phone users in the same area). My mobile data (LTE) was slow (about 1.5Mbps up and down). If it wasn't a Friday morning (when I was testing), I usually get 20Mbps up and 10Mbps down.

Verizon's argument

Verizon says that the people that download apps that crowd-source speed test data are "subset of all smartphone users that is likely to include more sophisticated consumers who are interested in monitoring the speed they experience when using their mobile device."

What does Verizon mean by that?

Most of our customers can't tell how fast or slow we are, so the crowd-sourced data isn't relevant.
It's a pretty good argument: The only people that would install the crowd-sourced speed testing apps by Ookla and OpenSignal are people that notice (the people that have used so much data that they've been de-prioritized in an effort to give the people that didn't use as much data speedier upload, download, and ping speeds).

It gets more complicated...

The month that the claims were made by T-Mobile was the month following the introduction of Verizon's Unlimited plan. Because this was a new plan, Verizon also states that this was the first time data-heavy users would have experienced network de-prioritization on the new Unlimited plan.

My question:

Wouldn't the same sort of sophisticated customers install the crowd-sourced speed testing apps as T-Mobile customers?