Estimate still seems way off?
Software cost estimation is tricky business even when all the variables are known (which we certainly don't have). One thing to remember is that COCOMO was created to model large institutional projects, which often don't compare well with distributed open-source projects. Beyond just development time, COCOMO is meant to include the design, specification drafting, reviewing and management overhead that goes along with producing quality software.
This model seems to be most accurate with mature, large projects. Young projects with little activity are typically overvalued.
То есть ядро Линукс такая модель недооценивает, а какой-нибудь MyFirstOS - переоценивает.
Никто никому ничего и не платил. Это просто метод оценки стоимости проекта по вероятной стоимости труда, необходимого для создания проекта.