Did you know a pound of feathers is heavier than a pound of gold?
Yeah, me neither. But apparently it can be argued because gold is measured in troy pounds, which have 12 ounces, and feathers are measured in avoirdupois pounds, with have 16 ounces, and since a troy pound is 372 grams and an avoirdupois pound is 454 grams, then a pound of feathers weighs more than a pound of gold.
I guess that’s kind of like saying its hotter outside if we measure in Fahrenheit instead of Celsius.
The reason for measuring precious stones, metals, and pharmaceuticals with a different system is accuracy. The troy ounce is based on the metric system and is an international standard. Although much more accurate now (because it has been better defined with the metric system, ironically), the avoirdupois system historically had a bit of a variance to it.