It would have to be the average length of a career, because it is absolutely verifiable the number of rookies drafted and signed as UDFA's every year and also make the opening day roster. However, what isn't included is the number of rookies that are picked up during the season. Practice squads are continuously evolving all season long.

Another possible factor is that a rookie signed to a 90 man roster and cut and never resigned has a career length of zero which is factored into the overall number. With that in mind an average career of 3.3 years makes perfect sense. If only 246 of 650 signed rookies made the week one roster in 2020 it means that 404 2020 rookies potentially had careers of zero years.