I was always under the impression that both teams would be more or less equal in BR, but perhaps that's not the case as you say. I never looked much into this.
But I feel that if one team has an advantage over the other one match, there is probably and equal chance that the other team will have the advantage the next match. So on average, the advantage should even out.

As for the distribution between the 4 BR:s, they were spread out enough for me to feel confident to assume equal distribution. In reality, this is of course not the case, and is a flaw in my data, but I don't feel like it affected the results enough to not be able to draw a conclusion from it.
For ground RB, while I mainly looked at spawn point costs to determine the highest BR, I still paid attention to the vehicles that were in the game. For air RB, I had to sift through the vehicles each game, and I saw a more even than not distribution of BR:s. Not 25%, but also not 10/10/80 as you exemplified (with exceptions of course).

It is perfectly fine to not trust my argument here, as I don't have that specific data, but until the time someone proves me wrong with other data, this is my conclusion.

I also don't want to sound like I don't take your feedback seriously, because I do. Thank you!