When doing a virtual run I can use my footpod stryd for power. However, Garmin doesn't use the footpod for distance/speed so my watch says 4:30/km but the treadmill + zwift show 5:30/km (correct because Zwift is reading my footpod speed).
I don't mind calibrating after the session but on my old forerunner 245m there was no need to calibrate manually as I could tell my watch hey just listen to stryd and all my values lined up.