That's an interesting insight to their methods.
We're not the most frequent cruisers, but have been sailing for decades. So, my experiences may not reflect the current methodology. That said, there used to be 1-2 "formal nights", depending on cruise length. There was a "Captain's Dinner" on the formal night; it usually highlighted their best cuisine - lobster tails. Therefore, that meal could not be in some standard rotation - it wouldn't sync. So, you'd see a rotation of 7 days, 9 days, whatever.
ggTexasGal, in this thread, said that NCL doesn't have those nights (any longer). That would free up their menu options. My last time on NCL was on Pride of America in Hawaii, about 9 years ago. Still had the tails back then, and, I believe, a 7 day rotation.
Our last cruise was on RCCL about 4 years ago. 9 days out of NJ. Had the fancy/lobster tail night. They sailed alternate 9 day and 5 day itineraries. Rinse, repeat. I would expect them to have 2 separate menus groups, one for each sailing. As for now on NCL, pure speculation by me, 21 days is too much variation (except on very long sailings). I would expect a set group of menus, cruise specific, and they stick to them, again and again.
Unless we get another poster here who knows more, I'll find out when we sail - because I'll ask them onboard - just for fun.
So, Pete, you're on the 1/12 sail?
What part of Canada are you from?