Evolution of code
The thing with any bit of code that has been around for a while, is that when change comes along, the tendency is to cater for the change by adding new stuff, while nothing gets taken away. Some stuff has definitely been taken away from this Date Dimension, but some historical artefacts no doubt remain.
All Date Dimensions are a blend of personal preference and business requirement, frequently with a bunch of history thrown in, and this one is no exception.
No doubt all Date Dimensions share a few common properties, but thereafter may diverge significantly. I