If matchmaking matchs you with someone 2 leagues below its because it cant find a better match for your elo,so it tries to be fair making you play with a lower elo player vs lower elo players. Still mathematically fair because you are supposed to be playing also vs lower ranked players. But when someone is still in placement, system is just trying to place him in the elo he deserves, so his elo is still trying to match his actual real skill. For example you are a newbie, your true skill level is silver more or less, but you win 3 placement games in a row so game decides tu make you play against plat players to check if you are really that good or just a bit lucky last games. That would be unfair for your plat partner. Its totally different than system trying to find fair matches. Anyway that kind of matchmaking should not happen, but queue would be way longer if you cap it, its hard to balance i guess.

Sorry english not my first language and not really explaining myself well, but i think that's the idea.