This question and answer is very closely related to your question. Basically, the IPUMS family interrelationship variables sometimes assign “unmarried partners” to be part of the first family unit. This is different from the family unit definition used by the US Census Bureau. So, since the Census Bureau actually publishes poverty values for the first family unit in the PUMS, the POVERTY variable in IPUMS USA uses these published poverty figures for the first family unit. This is explained briefly on the description tab of the POVERTY variable.