^ Yup. If you used two different chrony's to gather your speed data, then all bets are off. The only way to get meaningful data is to keep everything consistent, and only change one variable at a time. That's very difficult in the real world, when shooting, due to different RH, atmos. pressure, temp, load variability, etc. If you can keep everything the same, and just change elevation, you'd be able to see the true difference.
Try using the same chrony in both situations, and you'll have a much better idea.