Without speaking on behalf of Charlton Press and/or the Editor,* I might suggest that the statement in question was a logical conclusion more than anything.
Assuming you had access to a very large sample of new banknotes and could precisely measure the size of the notes, you could find the average width and height. You could also then plot the number of notes with varying dimensions. I suspect that the chart would look very much like a typical bell curve with very few samples with hugely variant dimensions and lots with minimally variant dimensions. At some point there you'll see huge drop-offs and you can conclude that there is where the official tolerances probably lie. But it's a lot of work and a nearly impossible task to get a suitable sample size for so very little payoff.
Either way, the Bank, like all manufacturers or companies having something manufactured on their behalf, have requirements for their product and tolerances to consider slightly imperfect pieces as acceptable. While we common folk may never know what those tolerances are, and while they may change from time to time, it is clear that the $10 notes with the missing circles commonly exhibited modest variances in cutting registration and banknote dimensions so as to produce half-circles, full circles, 1.5 circles, two circles and an infinite number of degrees in between. Because these were so common, it is reasonable to consider them to be within the tolerances of the Bank.
I don't think there's anything official to back up that statement. Presuming there is would be unadviseable, and presuming that there ought to be is rather unnecessary.
* I am in no way related to the Charlton Press. My statements are my personal opinions only and should not be interpreted as comments on behalf of Charlton Press and/or its staff and related persons such as the Editor.
Logged
BWJM, F.O.N.A.
Life Member of CPMS, RCNA, ONA, ANA, IBNS, WCS.
President, IBNS Ontario Chapter.
Treasurer, Waterloo Coin Society.
Show Chair, Cambridge Coin Show.
Fellow of the Ontario Numismatic Association.