If the baker gives you 13 and calls it 12, that makes it harder for a greedy baker narrative to stick. It doesn't have to be logical, it's about managing impressions.
> Who is even buying 12 loaves of bread when restaurants are rare and refrigeration non-existent?
The average family used to be the size of a small army. 12 loafs of bread could be eaten in 1 or 2 days if you've got 12 hungry kids and bread is a major component of their diet.