It is also possible that it could have to do with how you drink, and definitively how much you sweat. How one drinks may not have much to do with dehydration, but it seems to be somewhat related to how much one needs to drink to feel they have satisfied their thirst.
The following is anecdotal, and I am curious of what others know about it. I tend to take small sips very frequently while I hike; I swish the water in my mouth and must somehow coat or mix it with saliva. I drink more at camp, to make sure I have enough, but the frequent small amounts work really well for me while hiking, even in the heat. My daughter, and several others I have hiked with, seem to drink large amounts at once, pretty much guzzling it from bottle down the throat. Is it possible that the swishing and mixing with saliva, almost "eating" it really, could make a difference in how much water one needs and absorbs , everything else being equal. Could the same individual need less water if they drank it differently?
Another possibility could be breathing. I never, ever breath through my mouth while hiking, no matter how strenuous. I take deeper and deeper breaths through my nose, which also seems to help with pacing uphill and speed of recovery, including returning to a slow heart rate. Breathing through an open mouth may also add to loss of water.
Any thoughts on this?