No one has come up with an explanation as yet, but further to my earlier post, consider this:-
Water molecules contain one heavy oxygen atom and two light hydrogen atoms, so it weighs less than atmospheric oxygen, which contains two heavy oxygen atoms, and atmospheric nitrogen, which contains two heavy nitrogen atoms. These water molecules force heavier molecules to spread out, which reduces the air pressure, a reduction in air pressure causes changes in the drones barometric readings, so is it possible that when the drone took off, there was Less/More humidity (fog conditions at the time) later when you landed the degree of humidity present locally where you were standing changed, and so the drones instruments did not show that it had arrived back at the starting height....? just a guess I don't know but it is a consideration.
This makes sense however on my views its getting less obvious when operating several crafts at same the time - let me explain;
As I reported earlier, this summer I've been patrol flying with a friend who has a Q500 and my P4 ended with 2+ meters altitude offset when we tested hovering at 5m/ground, after both approx. 10 min. flight.
Not to mention same flight paths, sunny weather, no wind, 23c ambiant temp etc.
He's bird altitude readings were always rock solid and I could just notice he was overall less stable than my P4, but probably more to do with design. My friend told me my P4 was flying super stable, fast and smooth. But at the same time I was frustrated seing such readings on mine and wondering I should calibrate again.
I would be interested doing same with two P4 and not sure they would show same readings? It's complex apparently and probably only DJI can answer basis debug, logs etc.
Air pressure at sea level @ 15 Deg C =14.70 psi
Air pressure at 60 feet @ 15 Deg C = 14.67 PSI a difference of 0.03 PSI which equals 2.068 millibars, so it does not take a big change in either temperature or air pressure to effect readings.
I'm not saying there isn't another explanation of the problem, but as the other OP said how does it perform on a warm day with no fog...?
Funny you mention sea level; both I and my friend we were at sea level, large empty car park distant 400m or so from beach.
Would say my P4 performed bad in good weather conditions and could have been possibly worse with fog but no test case to share. Until now majority of reported issues are not in conjunction with fog condition, If I'm correct.
My friend said to me he never experienced altitude readings so far with Q500. Let me contact him to verify if still the case. Ill report feedback here.