Well generally typeface size is measured from the top of a capital letter (cap height) to the lowest point of the descenders, regardless of the units used for m

Well generally typeface size is measured from the top of a capital letter (cap height) to the lowest point of the descenders, regardless of the units used for measurement. And you don't even have to get to antialiasing and hinting to see issues with perceptual typeface size... Look across faces and you'll see wild differences in cap height, ascenders, and descenders relative to the x-height.