Well, rarity should have nothing to do with how a third party grader assesses condition. Those of us who are at least a little bit cynical know, however, know that the rarer and more expensive a note is, the more likely it is to be overgraded by the owner. There are not too many rare notes owned by modest collectors.
Grading standards do vary by series, even though they really should not. If we want to focus on BCS, it is my opinion that "they" grade 1937 series notes rather strictly, or at least more strictly than 1954 series notes. I can't think of a reason why this would be, except perhaps that it is starting with the 1954 series that this whole idea of flawed notes being desirable kicked in. What do I mean? Paper ripples are flaws. The Bank of Canada did not plan to make notes with ripples. Ripples are the result of wet paper drying. Ideally, for a 1954 series note to be considered gem, it should be lacking in ripples but displaying full embossing, and rippled notes should be unc-60 or choice unc-63 at best. But the opposite is true... someone decided that gem 1954 notes should be rippled, and flatter notes are lesser grades. And the Charlton catalogue went along with this backwards thinking because they know the average collector can't tell the difference between a pressed note and a flat but original note, so now your 1954 note needs to be rippled. By comparison, ripples are considered detrimental to the condition of Journey series notes. Journey notes need to be flat and unrippled to be gem.
The 1937 series does not have these ripple requirements. Maybe BCS grades more severely because they think these notes should show more signs of "originality", and the lack of ripples is considered a penalty, thus lowering the overall grades given to notes of this series.
Notes from 1935 and earlier seem to have a lot of "thumbing", resulting in dirty areas on the left and right sides of the note where counting creases are frequently seen. We know counting creases are allowed in uncirculated grades (except gem, of course), and counting practices in banks were similar right through the multicolour series. But later series just don't seem as genuinely dirty resulting from thumbing, so I think more tolerance for staining is extended to earlier series notes. I didn't work in a bank a hundred years ago, so I don't know why normal handling of notes made them dirtier than today.
