Public Rating and the hidden “Skill Rating” (MMR) are two different things. Someone playing BGs 24/7 can probably go 7K by mere brute force alone. Others who don’t go 7K may have a higher internal rating but because the system tries to also group people into public-ratings: you may only see them at the start of a season when you have not ranked yet.
Basically their prio seems to be “Match them by hidden rating, but try to not match high public rating with low public”.