What Speedtest does xfinity use for their “ Your personalized WiFi summary”?

My monthly summery always report 20% higher speed than what I’m paying for. My own speed tests, even when my laptop is directly connected to the gateway, shows about 20% less than what I’m paying for. What test is Xfinity running, and how can I run the same test?

I have an XB8 in bridge mode. I do not see any speed test option on the modems admin page or in the Xfinity app.