On 11/16/05, Felix Miata
Traditionally,
before scalable fonts came into wide use for screen media, screen fonts typically were available in limited sizes, commonly skipping odd sizes above 10pt (some common bitmap fonts still do that). The Windows 98 system menu font comes/came on only 3 pt sizes. 0-9, 10-11, & 12-up. 10pt is one such size smaller than the screen default 12pt (16px), about which: http://members.ij.net/mrmazda/auth/defaultsize.html:note1
I'd guess that it was a carryover from the hot type printing days. Movable type and Linotype/Intertype mats for typesetting only came in odd and even type sizes up to 12 pt., then even number sizes only after that. Some half sizes were available for text; e.g., "agate" was another name for 5-1/2 point, commonly used in classified ads set on a 6-pt base. 8-1/2 and 9-1/2 point were fairly common for newspaper and book text. I still remember the first time I laid my hands on the Addressograph/Multigraph Co.'s first phototypesetter. It allowed odd and even typesizes up to about 36 point as I recall. I was shocked, although it turned out to be pretty handy for filling column-width in display lines. It was also the first phototypesetter I saw that had a CRT rather than a 16 or 32-character LED for the display, but this was before WYSIWYG. The deluxe version came with an 8-inch floppy drive rather than a paper tape puncher/reader. Most of the text typesetting in high production plants was still being done without any monitor at all. If you worked in a newspaper, you had to be able to read 6-level Teletypesetter (TTS) code on punched paper tape (amongst other joys) to earn your journeyman papers back then. But I digress. Marbux