There are several instances of a league. Luckily for me, we’re still more than 15 x 8 players in the game! :)
The way it works is that you are matched against 14 other people that have roughly the same card levels as you, and preferably are in your league. But in the case we don’t find 14 perfect matchups for you, we sometimes fallback to giving you some higher or lower leveled players, in some cases even from another league (the latter should be relatively uncommon, but yes it happens, and I agree that it’s not optimal).
There is currently no guarantee that Player A sees Player B in his/her tournament just because Player B sees Player A. Hence, technically, it might happen that a player you saw finishing 4:th place, actually finished 3:rd or even won, from his/her own perspective, and gets bumped to the next league tier next week. This should answer the specific question @mvnla2 asked yesterday.