a meets/exceeds scale becomes stack ranking when the proportion of different ranks is capped so that even if everybody on a team did well, some would have to be given the lowest ranking
The main difference is that being in that bottom 30 is usually not “bad” under that system, ie it will have maybe a 0.8-0.9x expected bonus multiplier. Whereas under most stack ranking systems that call themselves as such, a D grade might mean that if you get another D you’ll be in hot water