I think MG's response is a good one too.
Other things to note: Who prints the notes too now may have a factor.
BAI and CBN are contracted to print money - and as companies (BAI private), they have to manage cost efficiency. Their methods have varied - from pulling notes with no replacements (as in BEU $10s and ARR $20s, and the recent high denoms), to inserting singles or sheets.
Their methods, we may never know. What we DO know is that those are done before they reach Bank of Canada (in most cases).
This is different in the past, where replacements were issued and inserted at the BoC level. Not sure if this would have had much of an impact, and not even sure when there was a change, but it just goes to show that many factors are involved.
I would say the fact that we have instant info these days, is one of the greatest factors.