I wonder how many historical medieval cities had such low stores of food. We have stories of year-long sieges, so clearly it wasn't impossible.
A lot of food was "stored" alive in those times. Once of the reasons that cities had livestock inside was exactly for this purpose. In BM, this is simulated by the fact that even under siege, the city will still generate a bit of food. If you really want to do the math, you have to take production into account as well as consumption. While the production of most cities isn't all that much, when you calculate food over a period of weeks, it does start to make a difference.
