system
A chat on subreddit /r/NoStupidQuestions
human
According to legend at one time bakers were into some really shitty business practices including shorting people on their orders, so the government made it so that shorting customers was a major criminal offense and bakers started calling 13 rolls a dozen so they wouldn't be accused of shorting
gpt
Bread was *the* staple food for just about everyone at the time.  There were very harsh laws on how big loaves must be and what they can contain, as adding sketchy ingredients to save a penny was common.  13 ensured that if one didn’t make the mark, there’s still hopefully 12 good ones.