Inconsistencies with speed and power

I just joined Zwift thinking it could be fun to try out some virtual racing. My intro ride went pretty well except I had some strange results when it came to the sprints. Not knowing about the sprint sections I was taken by surprise when the first one came up and didn’t initially respond to the “GO GO” that appeared on-screen. After a few seconds I realised what was happening and gave it some gas and got my first time for the sprint. Next loop and I was ready and jumped on the sprint at the line and gave it a fair amount harder, only to finish the sprint section with an ever so slightly slower time. My first thought was this is broken, but I continued on and thought I’d see what happened the next time round. Third time round and I went harder than the first but not as hard as the second and got a time in-between the two (all within half a second). After the ride I checked on Strava and the times for the 3 sprints are all pretty much the same but the power was markedly different

1st attempt: 35s, 42.3km/h, 296W
2nd attempt: 36s, 41.1km/h, 359W
3rd attempt: 35s, 42.3km/h, 340W

I am using a Tacx Neo and based on the “How Does Zwift Determine My Speed” post I would assume power would be the biggest factor in dictating my speed, looking at the numbers above it seems to me something is not quite right here. Any way to explain how averaging over 60W more over 35s can return a lower speed on the same section of road? (I know drafting is a factor in Zwift but I wasn’t riding in a group and don’t recall drafting a rider on any of the sprints. surely drafting couldn’t account for such a difference anyway?)

At what speed did you start the sprint.

If you start faster you need less power to increase speed.

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