The calendar was only required to see us thru to 6000; Without this assumption of 365&1/4 days {Julian calendar} despite a known 12 minute discrepancy, it would be billions of year from 1 Birkas haChamo until the next Birkas haChamo, rather than a 28 year gap.
Jewish calendar is so Makpid on having as few variables as possible as Patri said, that even in a Leap year which is Chosser, we still keep marChehvon & Kisslev @29 days, and Adar hoRischon @30 days, despite the weird resultant of having 3 consecutive Roshei Chodoshim @1 day followed by 2 consecutive Roshei Chodoshim @2 days.