Gold and silver, being precious metals and until 1837 strictly owned by depositors, had strict tolerance ranges. This was also needed because the coins contained their full face value in metal value. Copper on the other hand was coined for government account and while the intrinsic value was intended to be fairly close to the face value there was a built in seigniorage profit built in. This profit, in theory, was to be enough to pay the expenses of the mint. For this reason while the cent planchets had a specified weight, until 1837 there was no official tolerance range for copper coins. These coins were struck without a restraining collar so the diameters vary depending on the annealing of the planchets and the force of the strike. And since the strike was controlled by the force applied by the swinging of the arm of the screw press coining workmen every coins strike force varied. Hence the diameters varied. Why the unusual low weight for your 1821 and 22 I can't say, unless possibly the mint may have been trying to source a second supplier of planchets at the time. I know Boulton & Watt was the main supplier of copper planchets at the early mint after 1796, I do know they were trying some other suppliers in the 20's and 30's but at the moment I can't say if they did so in 1821,22 or not.