People are less likely to be married, and there are many more single-person households. As living continues to become cheaper, a partner is less necessary - we should expect marriage to keep dropping. In those single-person households, there are often fewer people working; many of the occupants are retired. The household income of the top quintile at a point in time in recent years was 17 times the average of the bottom quintile. The average family in the top quintile had more than two earners because there is often a teenager working. The average family in the bottom quintile had 0.45 workers because many of them weren’t working.
Correcting for that, it’s roughly four times higher.
When you’re looking at changes, not correcting that demographic change over time creates a misleading effect.