Reading through the DESCRIPTION for SERIAL, it notes:
SERIAL is an identifying number unique to each household in a given survey month and year. All person records are assigned the same serial number as the household record they follow. A combination of YEAR, MONTH, and SERIAL provides a within-sample unique identifier for every household in IPUMS-CPS; YEAR, SERIAL, and PERNUM uniquely identify every person in the database within sample.
I think this would perhaps benefit from making clear what is meant by “within sample.”