The editor of the Charlton book knows about the $2 S/R 2322690 from when it was reported as having appeared on eBay, and he also knows of another with an even lower number. The 17th ed. Charlton has been updated to reflect these realities.
The numbers in the catalogue have generally come from documents, both from the Archives (NAC Record Group 19) and the Bank of Canada. None were guessed at, pulled out of the air or calculated on the basis of chicken entrails, no matter how many people want to believe that they were. In general there was no information forthcoming on the asterisk replacement notes (there are a very few exceptions); these are indicated in Charlton as the lowest and highest actually observed and verified, and are shown with asterisks; resulting quantities issued are shown with (est.) when they are in fact estimates based on the low/high observations. Now, as Michael Zigler correctly points out, the figures obtained from documents are not accurate 100% of the time. (In fact, they are sometimes contradictory and it can take years of data collecting to determine which document, if any, is right (I'm thinking of the Purple Seal $1's of the DoC 1923 issue). They are usually right, but enough errors have been exposed over the past twenty years that it should not be considered highly unusual if another contradiction shows up. Such is the case with these S/R test notes, and also the Gordon to Coyne signature changeover in the 1937 $100s. In these cases all we can do is revert to the lowest/highest actually seen and try to narrow it down from there, knowing that we will probably never find the
exact changeover number.
As for the relative scarcity of the three S/R signatures, it comes down once again to the fact that the number printed isn't very important, it is the number which have been saved. Once it was discovered that there was something funny going on with that prefix coming out with different signatures, I suspect, more attention was paid to keeping them. Prices are determined for each edition from the data coming in from the dealers and collectors on the pricing panel. If there get to be too many of an item on the market, prices come down.