Zwift Measurements

Hi, Completed a few rides now on Zwift Island and really like it, makes the time on the turbo go quick and the miles fly by.
So i had a question originally on speed and understand there will be differences, overall the average for the ride matches what i’d expect.
I do have a question on distance traveled when comparing it to my Garmin. Each lap seems to be 0.1-0.3 miles longer than my comparison. Over short distances this isn’t a problem but when you put a few laps in the distance difference can rack up.
Should this be the case? I do get that there will be slight differences.

Your garmin is measuring how far your wheel thinks it has gone in the real world. Zwift is measuring how far your power pushed your mass around in a virtual world with virtual hills. They are very different things and the distances should be different.

Example, what if a world in Zwift was just 1 mile downhill? You could pedal for a few seconds on your trainer…start rolling down the virtual hill, then stop pedaling and let your trainer come to a stop. Garmin would think you moved about 50 feet, but Zwift would have you roll down the entire 1 mile and the in-game distance would reflect that.

Thank you for the answer, i do get the reasoning. However, if the route was a loop with a down hill and uphill back to the start. If no slip stream is available and the rider constantly pedals pushing the same amount of power wouldn’t it be correct to expect one lap to be the same, when compared to how far the wheel thinks it has gone?
I fully understand that the calibration between the 2 may be different and there will be small differences.
From an average speed view for each of my rides they have been about the same.
Tonights ride seems to be a lot closer than others so maybe its the data from my sensors. Great piece of software.