A more tenured owner might have a better answer, but from what I understand is:
Remembering that
DVC started in the 1990s before the internet, the different UYs are/were a way to spread out and encourage membership usage throughout the year as you would need to call in to make reservations. To me, 12 UYs is too many, 6 seems too few.
Not having a January makes sense because that would give an unfair comprehension advantage to those owners. Why not the other 3 months--

dunno.