David Slaomon: The Advanced TeXbook... ^^^^^
\speaker{Knuth} Ah. A lot of people are wondering about that one. Where you have `a minus c' or you say `x sub minus' or something, why is it that the height and depth are greater than the actual shape of the minus sign?. In fact, it's not just the plus and minus, it's also the $+$, $-$, $\pm$, $\mp$, $\oplus$, $\ominus$, $\otimes$, $\oslash$, $\times$ ^^^^^