I assume it's a convenient form of the holy grail of suppliers everywhere: price discrimination. If you can afford a big tip, and social mechanisms shame you i

I assume it's a convenient form of the holy grail of suppliers everywhere: price discrimination. If you can afford a big tip, and social mechanisms shame you into giving it, then great! The restaurant makes a lot of profit. If you can't, then the restaurant can still make a small profit off serving you anyway. As for why it'd increase over time... maybe that's just an effect of people getting a bit richer, but the marginal cost of serving a meal not so much?