What server is the router's speed test connecting to?

I'm getting WIDELY inconsistent speedtest results. Specifying the Richardson, TX Speedtest server that's hosted by AT&T, my upstream speed varies between 100Mbps and 900Mbps. The router's speedtest only varies between 800Mbps and 970Mbps. Now, you MIGHT be tempted to say, "It's a problem with your device!" Except the device is either a jump host running as a VM on a server that's cabled to a Cisco 4900 switch with a CAT5e ethernet cable, OR a NAS that's got a pair of 10 gigabit ethernet over fibre links into that same switch.

I just ran the speedtest from the NAS, and got a reading of 938Mbps. Ran it again about a minute later, and got 217Mbps. The router got 905. Given the consistency in test results from the router (which vary by at MOST 175Mbps), I'm VERY curious as to WHICH "Speed Test server" it's going to. Is it communicating with an ACTUAL Ookla SpeedTest server? OR is it using iPerf to measure network performance between my router and the splitter node around the block from me?

If it's the latter, then the performance test is disingenuous. It doesn't account for uplink bandwidth oversubscrption. It measures how fast I can send data to the entry point to the AT&T network, but doesn't account for my having to share that bandwidth with the rest of the neighborhood. And if this is the case, what's my minimum reserved uplink speed? I've seen my upload speed drop as low as 30Mbps, which causes serious problems when I'm on a Zoom meeting.