I recon myself that at the mint sometimes small runs of errors must happen (and not seen)in between die checks, or.... when a visual error is detected at the mint they would clean or change the die whilst scrapping X amount of coinage to clear the errors out of the system(missing a certain amount of visually hard to detect errors)
Then when the rest of coins get released into the wild through the normal channels they end up dispersed via time with few and far between..... but, sometimes still circulating within certain area's only (unless dispersed accidentally further afield unknowingly by travellers).... my thoughts there are that coins circulate via closed loops due to cartage and weight limitations as well as state border limitations .... like wherever they get taken to be rerolled for dispertion once again probably keeps them circulating in closed loops excepting for tourism (that's my take on it all)
So.. looking for an error that you have found within your closed loop area should happen easier for you (unless the particular error was walked into your particular loop from afar)if it was a small run of error that only ever came your way.