ZHVelo

I was going to already reply saying I agree with you after checking my first data for a flat (which is hard to come by here) but then I checked a few more and the results are vastly different. One ride I did 248W but only averaged 34.3km/h another I did 231W but had 35.7km/h.

So why is this?

I think it is mostly wind, because on the same ride as the latter data, I also did 229W for one somewhat flat stretch (+27m but also -27m) but averaged 37.3km/h and that was on the other side of the lake where often wind is from the rear (and when you come around on the way back home it is often headwind). That is a 1.6km/h difference on the same ride at almost identical power output.