Perhaps the tournament sub rules do help keep some games closer? I'm not sure why it would, but perhaps it's possible.
There's many reasons sub rules can change the outcome of a game.
Say you have a really hot midfielder that scores, is strong at distributing, and dominates at defense but they get tired 15 min in + need a five min break before they can go back on the field. Coaches could run 2 lines + whenever that mid goes off they switch to a 100% defensive lineup + after the hot mid rests the coach goes back to more of an offensive setup. In a tournament this can happen any time during the game. In league it doesn't work because once you leave the field you're done for the half.
Another popular tournament tactic is to sub all the time to keep all the players fresh. This also happens when teams are up on goals + trying to delay + burn minutes so the other team can't score. Neither of these would happen (with the same effect) in league play with more conservative sub rules.