Thanks, Sean, that's very helpful to know. I can see further evidence of this by running a multithread and a single-thread test while one TV is streaming. In that case, both tests show similar results, which is exactly what one would expect... i.e., the streaming connection plus the single-thread test together max out my contract bandwidth.
Great suggestion! Better yet, a screenshot of my router's WAN Traffic screen, since Task Manager can't show that other network devices are idle or off.
The problem is, they're going to ask me to run a speed test on their preferred site. I don't yet know which site they'll point me to, but as I mentioned in my original post, other test sites typically report close to my contract speed. For example, I just ran turned off everything and ran a TMN single-thread test and it shows 3.6 Mbs with middle variance of 47%. Next I ran tests at CenturyLink (see attached screenshot), Ookla, and Spectrum. All graphed consistent D/L speeds at 4.7 Mbs for the duration of the test. Is this because other sites are running multithread tests? I'm going to have a hard time trying to explain that one to my ISP!