Most notably how “administrative cost” are calculated as a percentage of overall expenditures. Medicare patients are older and sicker than the population at large which skews the administrative expense % artificially low...ie (total expenditures- claims paid) / total expenditures.

A better way to look at efficiency is average administrative cost per insured...when Medicare is fully burdened with many of the program’s costs that are born by other govt agencies (IRS, HHS) and many of the costs the program pushes onto private insurance plans...the actual cost per patient of Medicare is Higher than private insurance. Now there might be other cost/access benefits for Medicare for all, but the administrative argument is funny math.