Why are network speeds reported by Activity Monitor different than the ones reported by speedtest.net?

It's using a totally different averaging method.
It's not examining the exact start & stop point that Speedtest is, it's just doing an average over the past 5 seconds.

I got 217mbps Speedtest, 20MB/s Activity Monitor. It took it at least 2 updates to even reach that, by which time Speedtest had just about finished.

Switching Activity Monitor from its default 5 second update to just 1 second* & the values just about match - I was getting 217mbps Speedtest with 27MB/s Activity Monitor (roughly).

Also bear in mind you are comparing a one-trick pony - an app dedicated to providing line speed results - against one that's more a general overview of what your entire machine is doing.

After comments
As there was still a discrepancy at 1s sampling, try to eliminate sources of error - switch from WiFi to wired Ethernet; remove any VPN connections; test another account or Mac.

*View menu > Update Frequency