This would have been my first thought. Another thought is the difference in the effect of wage inflation between higher and lower incomes. I intuit that peopl

This would have been my first thought. Another thought is the difference in the effect of wage inflation between higher and lower incomes. I intuit that people on higher incomes are more likely to see rising salaries in real terms (either by growth-in-job; internal promotion; or selling themselves to the highest bidder). The other side of that intuition is that those on lower incomes are more likely to be stuck in dead-end jobs, getting whatever across-the-board "cost-of-living" increase their employer decides to award. If (as is often the case) this is below CPI, then the inflation in the cost of their basket of goods is effectively higher for them.