The BLS calculates discretionary income by subtracting all ordinary expenditures from income in a given decile. Which is ~$1,000/month for the median household. Some of that leftover money may be spent on non-ordinary expenditures, like the odd expensive hobby or lifestyle flex, which is also captured in this table but is not included in the cost of living an ordinary life. I find this to be an eminently reasonable approach to inductively measuring the cost of an ordinary life without making a judgement if some spending is excessive or luxury. Spending money on a car is an ordinary expense, the BLS doesn't care if it is a Honda or a Ferrari, both are included.

This explains another apparent anomaly as well: ~35% of households have no meaningful discretionary income, yet only about ~15% (per Federal Reserve studies) have difficulty paying their bills. Because there is a substantial amount of luxury spending padding the ordinary costs of living, households in the 15-35% range have significant elasticity in how much they spend in some of these categories.