Pace data exported to Strava not smooth?

Does anyone know why run speed from Zwift doesn’t look the same in Strava? Run speed during my zwift runs match what my bluetooth enabled treadmill says (there’s actually a few seconds per mile difference but it’s consistent - doesn’t jump around if I don’t change the pace - I haven’t done the math to see which is correct at a given mph). Looking at the run after on zwift website shows flat lines for pace when the pace stays the same, and it moves appropriately if I adjusted the pace during my run. So far, treadmill display, zwift stats in-app during the run, and zwift website all appear to be consistent.

When I look at a zwift exported run on Strava App sometimes the pace graph shows momentary spikes even though I knew I was going a constance pace at that time. The Strava website pace graph shows even more fluctuations. Usually 1-2 seconds per mile but sometimes as much as 10 seconds per mile difference. Those bigger spikes correspond to the spikes on the app. The fluctuations aren’t uniform.

I have a Sole treadmill - is it sending belt speed to zwift? I assume not because when I start a run it takes a few seconds to get up to speed but in the game speed changes look to be instantaneous and I don’t see the slow ramp up in the zwift or Strava data. Or is zwift sending something else to strava that’s different from it’s own website? It doesn’t seem to make a difference, Overall pace seems to be within a second per mile each run.

Just strikes me as weird. Does anyone know what’s going on?