i dunno, that's just how it's programmed. though if i had to field a guess, i'd say that the way the game calculates your score, winning on april 1 or earlier would result in an unrealistically high score. maybe. like i said, i don't know.
Based on my understanding of how game length depreciates score, this isn't the case.
I'd be glad to present math on it but I'm having difficulty finding an acceptable set of numbers to work with as far as scoring in turns 1-12 is relative to each other. This is probably because of my minimal experience with ZYW games.
My guess is that it's just an arbitrary point chosen by the developers when imagining what kind of timeframe it would take for the player to win. In this mindset, if it weren't 12, it'd be 11, or 10, or 6, so there's really not much complaining to do about it.
Only once, though.