Well, I've just learnt more about fonts than I ever really wanted to.

The issue was that the Australian Standard for legibility on a computer screen specifies certain minimum heights, but "height" for a font can mean many different things. For example:

• ascent (or cap height or H-height) = from the base line to the top of the highest flat-topped capital letter in the font (such as an 'H')
• descent = from the base line to the bottom of a letter with a descender (such as a 'y')
• font height = ascent + descent
• font size = font height - internal leading (the space used for accent marks or diacritics)
• x-height = from the base line to the top of a flat-topped lower-case letter such as an 'x'
• aspect value = ratio between font-size and x-height

(UPDATE: Oh, the Australian Standard specifies a 3mm minimum caps height, by the way.)

It took me a while to track down some good resources on these, so recorded for posterity:

• A good explanation of the various heights used to size fonts
• List of common font aspect values and x-widths
• W3C has an excellent summary of how to measure and describe fonts