It may be that your ISP, or another organisation that your traffic flows through, has implemented an efficient and accurate-enough way to identify speed-test traffic from amongst the actual video traffic. Possibly just by assuming that after a few minutes of transfer or a certain amount of traffic, your current use is not likely to be just for speed testing.

I assume this is why the tester does not run an upstream test by default: otherwise the lack of a block of upstream traffic would be a good signal that video playing is going on rather than throughput testing.