I think it's a factor that everyone has come across, but depending on what conditions they did their numbers on, they may not notice that much, and/or scope shift or gun speed change or even mechanical warpage can mask it, or make it worse.
You don't have to have done too many shoots over a few seasons to know that there's always a cold shoot that everyone seems to drop low on, and a hot shoot where the shots go high. Now some of that can be the gun, the scope, but if you sit on your own plinking range, with the same chrono and shoot the same target and your suddenly 1 MOA out it could be the reason why.
Thicker air over water? Possibly if the air is not moving and allowed to sink on it. I saw drop at Bisley one evening, nothing changed under the club house, but when shooting had stopped a measurement down the far end on the edge of the woods saw a big change. (mind you it was 10pm and had dew forming).