One rod being off vs another one that's dead on could very well be what the manufacturer weighed (or rather when). Just the blank, one number, built out another.
But for context the 'common penny' comes in at about 11 to the OZ 181/pound). Older pennies were at 145/pound. So the game here is the listing is 1/2 oz off, we're actually talking about the weight of 5 or 6 pennies.