Why does some of the food have a -number??

I am trying to figure out why with sometimes the fish, other times the cabbage, or both, have a minus number under the + number. So I am gaining food, but also losing, and I just can't figure out why.

Your people use resources. So do some of the buildings (like the mill uses wheat). For fish and cabbages in particular, your people are eating them.

When the amount being used is more than the amount you are bringing in from the fisherman cabin(s) and the cabbage field(s), the game tells you by putting that negative number on display.