But not all adopted kids are treated equally, as I've mentioned before. My friend, Marvie, was adopted two years ahead of his sister. The parents acted as if he wasn't theirs at all and it was entirely noticeable if you were around them very much.
There are lots of adoptees in my family, for whatever reason. My cousin adopted two kids, didn't like one and sent that one back.
I could go on. I will say again: bonding is tricky, not automatic, and not all parents bond to each of their bio-kids, obviously - in our family, my grandmother never took to me, but did find one of her adopted great grandchildren quite appealing (adopted by her favorite daughter's daughter). My other grandmother adored me (and her other 2 adopted grandkids).
We don't know anything about these various children or why some might be favored (or whether adoption has anything to do with it), but from the POV of most adopted kids, when they are less favored or abandoned or sent back into the system, they feel it has something to do with the initial abandonment, I think that's natural.
It's also natural for some people to prefer their bio-kin and for other people to take longer to bond to newly adopted kids.