People choose to spend more or less on food, eat different things, etc. If I want to spend a lot on food, that's my choice, but I should give up something else. Or if I decide to eat rice and lentils multiple times a week, I should be able to save money.
Even more important than fairness is just practicality. There's a huge amount of infrastructure dedicated to people going to stores/restaurants to buy food in the manner they see fit.