I'm not sure either because I haven't been able to ever get it to do that.
My guess is something on the provider end is messing with TestMy.net's output. It may be trying to accelerate previously loaded content.
I do a lot on my end to try to bust host caching and clear your cache programatically. But you may get an occasional outlier. Seems to really only affect the multithread download test.
You can delete that outlying result by checking the box next to that result and click the trash can at the top of the result details table.
When I'm not programming or building server infrastructure on the backend I often scan the results for outliers. After so many years of doing this and adjusting the program they're pretty few and far between but I would like to develop a solution for any remaining.
When I've rotated back into programming mode I'll make sure to make the necessary adjustments to prevent this. I think before the result is logged your average will be taken into account, it's already being queried at that stage. If the result is much higher than your average and the result is over 1 Gbps a re-test occurs with further cache busting measures. After re-test, if the second result is within a range of the first result it will allow logging. And if not, the test will die with an error so we don't create a loop.
When I find these in the results I usually see the client re-test soon after, the re-test always returns to the median.
It would really help if I could reliably make the issue happen but it's an intermittent issue I've never personally seen. Unless I witness it I can only try to fix it blind. But I think we have a roundabout solution, just need to program it in there.
Thank you for the constructive feedback, it's always very helpful. You are building TestMy.net.