Side by side speed comparison on flat, zwift vs irl

On my Tacx Neo I thought the indicated speed in Zwift felt low compared to what the same effort would produce outside. So I made a comparison. I found 1 km of flat terrain, on which I was going with comparable effort in zwift and the same irl. The Irl ride was measured with a stages left-side power meter. I calculate about 5% loss for drivetrain on the Neo, and you can see when comparing

power for both rides.The outside ride has about 5% more watt output. It should be noted here that there was very little wind that day. Or possibly a very slight head wind. As you can see. I was getting more speed irl. Which makes me think the zwift calculations are pretty conservative. Thoughts?

too many other factors to say really - road surface, is your power meter accurate, is your trainer accurate, is your left/right balance 50/50, etc

its close enough not to worry about i’d have thought and does speed really matter that much? if it is just for training and you know the power then is that not a better measure?

Well. I dont worry. It’s more an issue of why. I believe my test is on the safe side here. If anything I would have gone faster still under perfect conditions irl. So the question is how they model this in zwift?

Maybe cut out some of the guessing by using the Stages power meter as the power source for the Zwift ride as well?
Plus the obvious checks that your height and weight settings in Zwift are the same as IRL. And bike is similar weight/aero as IRL.

There are so many people out there that are convinced that Zwift show faster speed than IRL.

Zwift use a mathical model to calculate speed, and there are a multitude of models out there and every one of those claim theirs are the best. I think Zwift is pretty close to IRL. But even if it is not then it is still the same for every one using Zwift.

Same goes for IRL different roads generate different speed, even in a velodrome that is make for speed there are certain ones that is just faster.

I am one of those people that is convinced that I ride faster in Zwift than in real life. In Zwift, my cruising speed is 1mph or more faster than IRL. I’ve used both my CycleOps Magnus and a Powertap wheel for power with the same result. I think Zwift’s algorithm is also very generous when climbing