Re: How come a font set at a certain pixel size is not actually that size when you measure it's height? For example, a font set at size 24 pixels is actually only 16 pixels tall (cap height).

The font height is the size of the Em box that the glyphs are in. The letterforms have to be smaller than this to fit, and are designed to look optically similar in size to other fonts. The actuall difference depend on the type of font; Script fonts, for example, have letterforms that are much smaller than their Em box.