Zwift calculates speed based on a host of factors. Zwift isn't sending that speed through the smart trainer to your GPS. The GPS is likely getting that from the smart trainer. How the smart trainer alone determines speed doesn't necessarily correlate to how Zwift calculates it.

That's my theory. Maybe wrong.

Probably something to do with your garmin basing “speed “ on flywheel velocity, whereas zwift will take all those fake 60mph downhills into account

This is very much Apples to oranges.

Distance on a trainer is not given based og watts alone. Zwift has its algorythms, based on varying incline/The route you choose, The bike you ride, draft/no draft, etc. based of That zwift simulatorspill speed and distance.

Your Edge just has a flat algorythm, where it «assumes» you need x watts to go Y speed, and gives a distance based of that.

Edge and zwift do not talk to eachother.

This makes sense.