Think of how a baby acquires his/her mother tongue? When they are born, the baby doesn’t speak any language however the people who regularly interact with them helps in their language acquisition.  The more the people who interact with the baby, the faster and better the baby learns to speak. This is how even you learnt to speak right?

Now imagine you moved to a remote village in Japan or China where English is hardly spoken. You don’t know Chinese but in a matter of few weeks or months you will learn to speak basic Chinese to survive. This is because you are surrounded by people who speak only Chinese. This is Speaker Variability in real life.