I believe that the writer of Genesis tried to bolster the thought found in Psalm 90:10 (both Genesis and that Psalm alledged to be written by Moses who supposedly lived over 120 years) that our lives can only reach 70 to 80 years unless God blessed us with extra life. And then the writer shows that God did that for the men of their fictional history. That way, such men could be viewed as "larger than life." But because it was fiction that could not be matched in the days of the reader, those lives were shown to be getting shorter and shorter.
There is no way the stories are based on real people counting years in a different manner. Everyone knows what a year is- going thru one summer and one winter, or going from one fall harvest to the next fall harvest.