Why would my WiFi connection be faster?

Sorry, first post here -- I can't figure out how to delete.

Anyhow, in case anyone should read this far, here's my question. I am running an Eero Pro connected to directly to an RCN Arris cable modem. Rest of my system is three little Eeros. In theory, I've got a gigabit connection and indeed for months that's what the Eero speed test reported. Now it's reporting <100 Mb. Nevertheless, my network speed via the Eero remains high (500-ish). Any idea what gives? Thanks!