I just discovered the Ookla speed test app for Windows. I installed it and started checking it out. Much to my surprise, the speeds that I was seeing in the desktop version were quite a bit different from the speeds that I see using the browser version in Google Chrome. I have the 1.2 Gb speed from Comcast. The speeds that I’m seeing in the browser are significantly less than the speeds I’m seeing from the desktop version. I have tested this a number of times that I always see this difference. So does this mean that the speeds are I have seeing over the years in the browser are not the speeds that I actually have.? I am attaching a couple of screenshots to show the differences from running the tests. So for speeds that are in the 500 megabits/sec and up are not accurate in the browser? Comment are appreciated.
Its possible network differences, a way to sort is check from the command line…
Speedtest CLI: Internet speed test for the command line
Thinking your Chrome test is an anomaly…I just tested thru my Chrome & got 347Mbps…thru Firefox 342Mbps…then tested thru my Ookla app & got 349Mbps… 
Your test results are pretty much the same as mine when I had 300 Mb/s download speed. My theory is that for speeds that are more than 500 Mb/s and higher down, the browser Ookla speed test cannot keep up. I have tested chrome, edge, and Firefox with the browser and I get pretty much the same results with all 3.
I consistently get the 925-975 Mb/s results with the windows version of the Ookla speed test. I would be interested to hear from others that have higher download speed plans with 600 Mb/s and above.
Copy that…Ookla SpeedTest supposedly added a 1 gig connection for testing 5G phone speeds…but I don’t know if they did the same for computers…I would think they would’ve by now, as 1G + has been out for quite a while…but with my speeds, that scale wouldn’t pop up when I test. Like you say…maybe others with super duper high speed service will chime in