I think that's the point. Supplementing up to a given minimum has counterintuitive incentives, causing the program to pay more in aggregate than if it had loose

I think that's the point. Supplementing up to a given minimum has counterintuitive incentives, causing the program to pay more in aggregate than if it had looser caps (and linear scaling) and let more people work and earn slightly higher incomes by reaping the benefits of the program.