Accuracy of "Data Sent/sec" in Activity Monitor on MacOS Catalina
The main reason is that Activity Monitor shows the sum of data sent/received on all network interfaces, whereas your router only shows the data actually routed through it.

This means that there is traffic that your router does not see, and thus does not count into its statistics. This could for example local area network traffic directly between your MacBook Pro and other devices on the local area network reachable without routing. Another very common example is the loopback network interface, found on all Macs, which offers communication internally within the same computer (i.e. between different processes/programs running on your Mac).

This means basically that you cannot equate a number from the sum in Activity Monitor and any sort of "my internet connection is used at this speed".

If you want to figure out what exactly is the cause of the discrepancy - i.e. even if you say that you're "reasonably confident" that there's no other data transfers, there actually must be other data transfers - you can identify them by measuring or sniffing the traffic on all your network interfaces seperately. An easy to use tool for this could be "Wireshark", which will allow you to sniff, dump and do statistics on data transfers on all network interfaces, including WiFi, cabled ethernet, VPN virtual interfaces and loopback interfaces.

The only explanations that can fit your observations, if you're correct that there are no other data transfers, are that either (1) Activity Monitor is flawed and does not display the correct values, or (2) your data transfer comes with an extreme amount of overhead. Both explanations seem very unlikely.