the sensor on the hub assumes you are riding on a flat road - zwift takes uses your power, the gradient you are currently riding in game and if you are drafting others (among other things) to work out your in game speed.
imagine driving your car and not moving your foot on the accelerator if you are driving on a perfectly flat road your speed would be different to when you are going up hill or downhill or following a car and drafting them etc. it’s a similar thing.