Because the chief (the one holding the resources) their troops are consuming more food than the town makes.  There are things you can do about this, like get equipment that gives bonus for food production, buy through the clan store or the store to buy with gold food production bonus items for 50% or 100% increase and resource production bonus items of 50% or 100%, these both can be used together.  Buy from the store items that reduce the amount of food your troops consume.  You can also set hero skills for food production bonus and do knowledge upgrades for food production bonus.

I find in most cases the food economy is not necessary and is the most expensive way to resolve the problem ... that is why I also explained other options.