This variance between prisons was true even in the High Middle Ages. In contrast to the typical bread and water diet, some places offered animal protein and fat. From Captivity and Imprisonment in Medieval Europe, 1000 - 1300 by Jean Dunbabin (p.122):
In Cambrai, arrangements were more lavish for debtors, with meat, herrings and cheese as well as soup. In this case, it was the jailer's responsibility to provide the sustenance; but the cost was all passed on, either to the debtor himself if he managed to secure a loan to repay what he owed, or to the creditor who then took charge of him.
The source is Le droit coutumier de Cambrai. If you're already in prison, you might as well go into greater debt and be nourished and vigorous than languish on bread and water.
So remember folks, with such great variability, you'd better do a little research about prison diets before you pick which region to go on your violent rampage in.