I don't have time to read 5 pages of responses since I'm heading out the door to work in a minute, but I'll give you the short version that most don't understand, and even fewer would acknowledge.
FD manipulate the BGS overtly, all the time, and that's absolutely fine. They have to in order to progress things like CGs, core narrative, Thargoids, which factions appear in a newly founded system after a CG, etc. . Back in the day, a good deal of CGs were player-submitted, so it was quite literally a popularity contest to put together a CG for FD to run with. I'd personally love a Thargoid attack against my home system one day... but I know that's pretty unlikely. But other players have also said they'd hit the roof if FD attacked "their" systems unannounced with Thargoids.
And this is the kicker... although FD have acknowledged the growing player base who directly manipulate the BGS, the BGS does not exist to provide a group-vs-group strategic conflict game. That's what Powerplay is for. The BGS is, simply, the backdrop against which we all play. It has to be, otherwise FD cannot run any more narrative events. The fact we use it as a strategic game of chess is actually just a secondary effect, and FD have on many occasions said that if people played with the BGS effects at the forefront of their mind, they've done the BGS wrong... I think they've said that on three occasions now and are yet to directly contravene that.
Other posters already said that Sirius got three settlements because they're Odd settlements and that's the mechanics of any conflict... so that's your actual reason. As for this "rabbit hole", there's no consipracy or employee malice at the end of this. FD have to and do manipluate the BGS to drive their narrative focus, and that is absolutely not beholden to the activity of players. It's a fundamental piece to understanding the BGS and it's design decisions (and a big reason why we don't have "direct control" over factions like so many ask for).
EDIT: As for conspiracies about FD having a bias and railroading CG outcomes, which many have made mention of... that's a thing, but it's probably not intentional, and just a byproduct of bad design.
EDIT: And just to be clear, this isn't a suggestion FD randomly interject into random conflicts because "Oh, we want this random faction to win". Only that BGS manipulation as an activity outright does exist, and any "BGS Play" is secondary to that.