With an internet connection, you’ll see lower speeds of actual payload throughput due to protocol overhead and error correction. Generally with a 1000Mbps connection, you’ll see a max of about 940Mbps.
Then also, speed test servers can be busy, the testing device can struggle with full “wire speed” gigabit connections, etc. So if your eero is saying 800-900Mbps, that wouldn’t surprise me, and I also would expect some variability.
If you connect a computer with gigabit Ethernet and test at speedtest.sonic.net, you should see >900Mbps. But again, some computers cannot keep up, USB dongles can be slow, and antivirus software can have a huge impact.