Font size numbers seem so arbitrary. The number includes some padding at the top, and some padding at the bottom. The bottom padding is sometimes occupied by the descender of letters like 'g', 'j', 'p', 'q', or 'y'. The real measurement to look for is the Cap Height, or the height of the typical capital letter. Even though that number will leave out the space of descenders.
It will also vary, often inaccurately from typeface to typeface. 12pt is supposed to be 12pt (6 lines per inch) and going between two differing typefaces, you may get upwards of a 50% variation. That doesn't even account for interface scaling to physical size, which no major OS does right, and only phone devices tend to come close.
