Well, I would say that the name on the bottle is representative of the methods used to produce it. If a brand has a good reputation, chances are they are consistent, have a history of strong operations and client support.
You are right that product A, procured from the same AAS raw source and produced the exact same as product B, will be the same in the end. In reality, I don't think that this scenario replicates itself in real life.
It's easy to see two vials and think, "these look similar". But differences in production, QC, dosing, and client support all make a difference.
Do you feel me?