I wonder about those "controlled conditions"... I know its anecdotical evidence, but this really contradicts everything I have ever seen after years ans years o

I wonder about those "controlled conditions"... I know its anecdotical evidence, but this really contradicts everything I have ever seen after years ans years of interacting with immigrant families (and being a immigrant myself). Kids are usually perfectly fluent after a couple of years of school while parents are still struggling, even if they are also totally immersed. It really surprises me this is considered a "myth".