Why is the quantity of a bakers-dozen 13?
One for the devil to spoil. That’s what mom always said anyway.
In England if the baker was caught shorting someone, they could lose their business. So they started the habit of tossing in another roll to ensure that when the package got back to the manor, there would still be twelve.
The explanation I heard was that during the Middle Ages, weights & measures laws (including the standard weight of a loaf of bread) were strictly enforced so to avoid complaints a baker would give customers an extra loaf if they bought a dozen.
You got ripped off, tell them you want 14 rolls (or oysters, or whatever).