I'm not sure they'd be the ones on the lower end of things because when you don't earn much, you don't want to risk what you already have for almost no increase

I'm not sure they'd be the ones on the lower end of things because when you don't earn much, you don't want to risk what you already have for almost no increase (given 5% of little money is very little) whereas 5% of a medium salary is much more of a difference/incentive.