Why is a bakers dozen 13 and not 12?

Answer 1

Bakers traditionally included and extra 13th piece in goods sold by the dozen, implying generosity. In fact, goods sold in dozens most often were those that didn't 'keep' well, and couldn't be sold as 'day old' the next day.

Answer 2

Bakers sometimes scammed people. Thus, to prevent people from being angry, a baker would give 13 instead of 12 things so people wouldn't get mad.