perth mint strikes 10 kilo silver bullion coin, page-17

"The Troy ounce is the traditional unit weight for precious metals, believed to be named after a weight used at the annual fair at Troyes in France in the Middle Ages.
Conversion:
1 Troy oz = 480grains = 31.1035grams = 1.09711avoirdupois ounces."

http://www.goldbullionaustralia.com.au/preciousmetalscoinsfaq.html

So Kenpitt you have an exception to the rule then.