that they are looking at their eye to the top of the others head.In other words they are adding the differance of their eye to the top of their head to the others persons height.
Im 5-8, your 6-0.If i look at the top of your height im forgetting about 3" of my own height.To get a true estimation of your height i would have to look you at eye level.
Everyone is the same height on the floor anyway,
Westside.