I can understand your confusion.
All I can think of is that some rats as in people need the strength of others to help them be stable. If these rats are with weaker rats, it "freaks" them out. My guess is that the rat in question is not an alpha rat but needs to have dominance over her to keep her straight and stable.
Whatever the reason, it's great that she finally has buddies.
I know that some rats can't get along with just any other rat. It takes a special group, or even circumstance before they are ready.
Radar was one such case. I had a hard time finding the perfect buddy for him until I thought of Isabelle, which at that moment made perfect sense to me. Those two are such lovebugs now.
A word of caution though, for those not fully observant of rat behaviour, what OldsGal did is I'm sure with great awareness and she was at the ready in case something happened.
In most cases, putting alpha with alpha will not work.
I remember reading about a "so called" breeder, not the one we all know of but another Canadian one, she had a very mean alpha male, he lived alone. She then either acquired or bred a younger male who was being mildly aggressive in his cage of other male buddies. All these rats were not neutered cause as well all know, breeders don't believe in speutering. This breeder thought she was going to teach the young one a lesson by putting him with the mean alpha. You guessed it. Both males went at each other, both cause such severe damage, they had to be rushed to the vet to be put to sleep.