Except in the case of Freeman of course, who actually studied a massive random sample from circulation ; while not perfect, his estimations of comparative rarity are as accurate as will ever be achieved for the bronze coinage. In terms of actual numbers of exceptionally rare coin varieties that were barely or not represented in Freeman’s sample, there will always be an element of flux as demonstrated by RS’s excellent ‘Rarest Pennies’ register. A ‘unique’ ‘slender 3’ 1863 penny becomes three known for instance, and more will be found. And in the end, it is market availability and demand that determine the price.
In general I can see where you are coming from. One has to hope that those declaring, and particularly publishing, rarity values make every effort to ensure reasonable accuracy. You will not find a ‘rarity’ value on any of my coin tickets, for good reason.
Jerry