Immediately posting an answer here - the issue was in my lack of understanding of the documentation.

Rather than - as I assumed the documentation meant - block groups being to the power of 3, 5 or 7 (i.e. 1^3, 1^5, 1^7, 2^3, 2^5, 2^7, etc.), what it means is groups in which the base value is 3, 5 or 7, raised to a power (i.e. 3^0, 3^1, 3^2, 3^3, 3^4.. 5^0, 5^1, 5^2, 5^3.. 7^0, 7^1, 7^2.. etc.).

So I guess that for any given block number, if the nth root is a 3, 5 or 7, it'll have a backup super block and descriptors at the start. This may have been obvious to some, but the wording of the EXT4 documentation was a little ambiguous for me, hence the confusion.

Immediately posting an answer here - the issue was in my lack of understanding of the documentation - and lack of appreciation of the subtle difference in the used terminology ("a power of x") vs. what I read in my mind ("to the power of x").

Rather than - as I assumed the documentation meant - block groups being to the power of 3, 5 or 7 (i.e. 1^3, 1^5, 1^7, 2^3, 2^5, 2^7, etc.), what it means is groups in which the base value is 3, 5 or 7, raised to a power (i.e. 3^0, 3^1, 3^2, 3^3, 3^4.. 5^0, 5^1, 5^2, 5^3.. 7^0, 7^1, 7^2.. etc.).