There isn't anything more complex...aside from the fact that not too long ago, there was an promo running where you would get double the normal amount of crystals for reborning. That multiplier wasn't shown on the reborn screen, all the numbers were just double what they'd normally be. That's probably what's throwing you off, because there's no diminishing returns on reborning - the number shouldn't go down like that unless you had a temporary multiplier that expired. Some cash shop purchases can temporarily increase crystal yields, too, and that's not shown on the reborn screen either. Same goes for moetan and moetifact bonuses.