I thought I'd share some of this random crap with you in the form of another acronym-ific series. I give you - Word Origins from Left Field - that's right, the WOLF (oh, how clever is she? She made an acronym out of her agency's name!) Er... ignore the fact that the "from" doesn't fit.
A baker's dozen actually equals thirteen. Now why would that be? Turns out bakers weren't the most trustworthy of shopkeepers back in the day. Air pockets can slip into loaves of bread, and it seems that some bakers took advantage of this, charging full weight for bread that was a little light in the loafers (knee-slapper!)
This was such a problem in England that Parliament passed a law in 1266 regulating the weight of bread, the penalty for shorting your customers being that you were nailed to your own doorstep by the ear. Uh, yeah. Shopkeepers decided that was a line they didn't want to cross, but there was no way to be sure that their loaves didn't contain an air pocket or two.
In order to stay within legal limits as well as assuring their costumers they weren't being shorted, it became common to bake thirteen loaves of bread, using the extra 13th as a "bonus" loaf. When a customer bought a regular loaf of bread, the baker also cut a chunk off the 13th loaf, to make up for any air pockets inside the first loaf.
Fascinating stuff, eh?