I have no idea how Zwift conducts its FTP tests.

If they have you do a 20 min test, your FTP is typically set at 95% of the average power for the 20 min test.

If its a 1 hour test, your FTP is 100% of the average power for the 60 min test.

There is a little more nuance to it (it depends as well on whether it was a "good" test, you can usually assess this by considering your perceived exertion, HR and the "steadiness" of your power output) but that's the gist.

However: if your weighted average power for the interval is quite different from your average power for the interval, this means that you did not have a "steady" power output but instead a "surge-y" power output. "Surge-y" power = bad test. The exception to this would be if you are looking at the weighted average power for the entire workout, and the entire workout includes warm up, cool down and other bits of power output besides the FTP test interval itself.