"We realize that the substantially larger amount of AT&T test takers may unfairly contribute to an overall lower download speed —so take this considerably lower result with a nugget of salt."

"The test results rely heavily on good faith that participants were being honest (and not, for instance, running the test over Wi-Fi)."

Two quotes from that articly in relation to the results posted - and the last quote:

"We acknowledge our speed test is not scientific"