Does your husband like to sit up late reading, playing chess, etc.? That sort of thing increases the light and coal bill quite a bit.
The pennies given for charity, church collections, etc., are also "extras."
Returning little courtesies—very often to "his" people—such as sending flowers, books, and occasional lunch or matinee, etc., etc., all make quite a hole in the housekeeping money.
The wear and tear of household utensils, linen, etc., means constant replenishment of one thing or another. A man may realize that his buggy or motor car has to have certain parts replaced once in a while but he is not apt to think of the pots and pans of the household side of things unless reminded.
It is a good plan to keep a few simple medicines at hand in case of sudden sickness, also a few bandages and the usual dressings required for accidents. Does your housekeeping money make provision for this?