Xfinity and AT&T disagree on what my internet speed is

My internet connection has been sluggish lately.

When I go to

https://speedtest.xfinity.com/results

Xfinity says my internet speed is 924 Mb/second

etc, etc...

When I go to any other place to test my speed

https://www.att.com/support/speedtest/

or

https://speed.measurementlab.net/#/

They say my internet speed is around 300 Mb/second.

What causes that discrepancy?