Segment times - speed vs watts

question for Zwift, why are segment times based on watts rather than speed?

Reason for this question is after doing the epic kom again last night in stage 3 of ToW there is a downhill section around 1.2km from the finish line, where you can supertuck, going 60kph and 0watts, my estimated time to finish the segment was 46 minutes, then once the gradient kicked in again and I had to actually put some effort in, 31kph and 240watts, my time dropped to around 24 minutes

surely a segment being time based it should be the faster you are going the quicker the segment is completed rather than the amount of watts you are putting out?

whatever timing they use, if it was inaccurate it would be wrong when you crossed the banner, surely? i spend very little time free riding on zwift but i don’t think i’ve ever arrived at a banner earlier or later than the eta.

That’s something that has annoyed me ever since they first introduced the leaderboards, the eta’s fluctuate wildly unless you’re putting out a steady power output and not drafting off anyone.