Many things go into determining your SR gains or losses, most of them are not PBSR. You can find a lot of details and sources in this post:
Back when streak bonuses were a thing, smurfs could get 100+ for a win after a few games but that is gone now and you have to be stomping really, really hard to even get close to 50. Unless you just leveled a new account or didn’t play for some time, because then you’d get bigger swings until you settle back into place.