When and Why did the Catan number distribution change
Having spent a bit more time with the rule book and the internet I've come to the conclusion this is a combination of two factors.


The image supplied above from the 5th edition rule book is a beginner set up that has no relation to the letters on the back of the number chits. It is purely a "balanced", and non-blocking starting point allowing new players to get into the game without too much trouble.
There's a printing error on the Chits in my copy (no idea how wide spread it is).


The printing error has two separate sources of verification.
This Distribution Question include a list of the Letter to number combinations that match my older set and (really tellingly) match this image from the Almanac of the fifth edition.



The numbers I have are in a slightly different sequence such that one 6 is swapped with an 11 (I and P), and one 9 is swapped with a 5 (M and O). Which leads to commonly paired 9s and 11s. Given the image Q above, this is probably a printing error.