[acheron]

This is the reason for the "bakers' dozen". Bakers were at one time required to sell a minimum amount by weight in a dozen, and so would throw in one extra item to ensure the minimum was there.

Ah, here it's on Wikipedia:

"In the United Kingdom when selling certain goods, bakers were obliged to sell goods by the dozen at a specific weight or quality (or a specific average weight). During this time, bakers who sold a dozen units that failed to meet this requirement could be penalized with a fine. Therefore to avoid risking this penalty, some bakers included an extra unit to be sure the minimum weight was met"