Dex’s article on the origins of the phrase “baker’s dozen” was an interesting read. As a baker though, I have a bit of a conjecture as to where the term comes from.
My theory pins the origin to simple spacing. Most cookie sheets are something close to a 3:2 length/width ratio. Consider, say, an 18" by 12" sheet:
We can fit 12 cookies of 4" diameter onto the sheet with this pattern. This is an even dozen – numerically convenient, but most bakers prefer to economize on space. Taking a cue from some of nature’s finest and most efficient foodmakers, we simply adopt the honeycomb pattern:
That’s 13 cookies in the space designed for 12. Over the course of a day or week, that’s hundreds of extras for no additional effort. When we’re tallying our goods for a shipment, we usually count by sheet – one, two, three dozen, since “dozen” is as good a word for it as any, then multiply by 13. Hence, a dozen to a baker is 13 in this regard.
I can’t say for certain that this is how the term came into being, but it seems as good a reason as any, and is used regularly by bakers today.