In our culture's case, in general the amount each one ate doesn't matter -- whether one had steak and the other just a salad, the salad person can very well still go on and insist on paying for everybody else. Nobody would care about "a 15% to 30% difference in cost" (the person paying for everybody wont feel it's a burden, and the persons having their meal paid, wont feel like imposing).

It's not about achieving some perfect balance or fairness in any particular meal -- people genuinely want to treat others, and enjoy taking turns doing so.

It's only longer term one would care for patterns of behavior (the person who after so many meals with others never volunteers to pay etc).