Cost of living doesn't increase linearly with the number of people in a residence - usually it's logarithmic (so two people per house is cheaper than one, which is more expensive than three).
Because two single programmers behave completely different than two programmers that happen to be married and have children. Different taxation, different activities, different emphasis of what is important and what is not.
I cannot stress enough how different my life was when I was single vs a married family man.
Why do people constantly group "married" and "has children" into one change, and then assume that it's being single that's the strange case? A two-income household containing two roommates vs. a married couple vs. an unmarried couple in a stable relationship don't look all that different economically. Adding children is what causes all the rebudgeting and tax-breaks and lifestyle changes. You can get away with just doing cost-of-living analysis once for "median parent" and once for "median non-parent" without just getting noise.