genejockey 

When I ride IRL, there are places where I go hard and places where I dawdle, and of course some where I coast. On Zwift, I coast a lot less frequently and I dawdle hardly at all. And there are no stop signs, or traffic lights to wait for. So, riding on the road is just more varied, and gives you more times when you can rest.

This is especially true in races. I don't race IRL, but I've been watching race videos from NorCal Cycling and others, where they display the speed and power during the race and in a real race there seem to be a lot of times when, even for just a few seconds, the power is zero - curves for instance. In a Zwift race, my experience is that there's no point in the race where you can just stop pedaling, even for a second, without getting dropped and having to fight like hell to get back on. I just looked at the last race I did, and there's not one point in the whole race where my cadence dropped below 55. No rest at all. I'm pretty sure this is because there aren't really curves on Zwift in any real sense. Sure, your avatar is going around corners, but there's no change in how you pedal, even for the tightest virtual hairpin.