An ounce is 28.34 grams The first coin weighs 31.1 grams Second coin weighs 31.21 grams So they weigh more than an ounce
There are two different ounces. An AVDP ounce, of 28.3 grams, as you stated. Then, the Troy oz, which is about 10% bigger at 31.2 grams. Old US silver was avoirdupois ounces of 28.3 grams and 90% silver. Silver was a dollar an oz, and the mint made 10 cents on each. I dont know where the Troy oz came from and why its used now. But the ASE is worth much more than a buck, and the mint makes much more than 10 cents each. Since it doesnt circulate, it doesnt matter
20 pennyweight in a troy ounce. 24 grains in a pennyweight. 480 grains in a troy ounce. 5760 grains in a 12 ounce troy pound. Avoirdupois has 437.5 grains in an ounce. 16 ounces to the pound 7000 grains. I think pennyweights are more precise for weighing gold. They use the 31.1 gram figure these days, but it is not as precise.