It turns our that T-Mobile spent some efforts in service management facilities to detect when a customer accesses speed tests sites, so a speed limit (for customers exceeding their data plan cap) could be temporary removed, and "non-accurate" performance information would be shown!
The Federal Communications Commission announced that T-Mobile US has ".. agreed to take steps to ensure that customers who run mobile speed tests on the carrier’s network will receive accurate information about the speed of their broadband Internet connection, even when they are subject to speed reductions pursuant to their data plans.
[Related post - "T-Mobile to Shape Misusage Over LTE" - here]
These speed reductions are specified in T-Mobile’s agreements with customers,and T-Mobile customers do not receive overage charges for exceeding their data caps. In June, T-Mobile began exempting the use of certain speed test applications, which allow consumers to measure the speed of their Internet connection, from customers’ monthly high-speed data allotments.
|OOKLA's SpeedTest Report - mobile carriers, US, November 2014 data|
Currently, customers who have their speeds reduced after exceeding their monthly high-speed data cap cannot easily understand the results of exempted speed tests. When these customers run speed tests that T-Mobile has exempted from data caps, they receive information about T-Mobile’s full network speed,and not the actual reduced speed available to these customers at that time.
The FCC was concerned thatthis could cause confusion for consumers and prevent them from obtaining information relevant to their use of T-Mobile services. The FCC and T-Mobile have agreed that T-Mobile will begin implementing the agreement immediately and will fully implement it within 60 days.
See "T-Mobile To Improve Disclosures for Consumers Using Mobile Speed Tests"- here.