Why cai_for_gene method substracts one from cai_length in biopython

Question

[yamasakih]

I am learning how to calculate CAI and how to implement it in Biopython.
I don't know why cai_for_gene method in CodonAdaptationIndex class subtracts 1 when calculating CAI.
Can someone please tell me why? Thank you.

https://github.com/biopython/biopython/blob/master/Bio/SeqUtils/CodonUsage.py#L157

Biopython: version 1.78