@35 RWE: Your explanations remind me of a common riddle we used when I was in high school 55+ years ago:

What weighs more? A pound of feathers or a pound of gold? Usual reply was that a pound is a pound so they both weigh the same. Wrong. Gold is measured in troy weight (12 oz = 1 lb) while feathers are measured in avoirdupois weight (16 oz = 1 lb). So, although feathers are lighter than gold, 1 pound of feathers is heavier than 1 pound of gold.