I find that many mistaken someone being an introvert versus someone being a shy individual.
A quick story of Irene, with her permission, of course:
One day, when she was around 5 years old, Irene was walking outside with one of her guardians. Suddenly, they stopped, and her guardian started conversing with an adult that Irene did not recognize. The adult talked to her, but she did not respond any more than saying "Hello." in order to not put her guardian "on the spot".
She did not wish to speak with the adult, since she found them extremely untrustworthy (due to her excellent intuition). After a bit more of silence on Irene's side, the adult said to Irene's guardian: "She is shy, isn't she." Irene can tell that the adult was feeling awkward and slightly embarrassed. However, Irene most certainly was not and is not shy. She does not talk much to those she is not familiar with (yet she talks a lot about things that interest her, with her loved ones). She does not fear social interaction or get nervous because of it. She is simply selective about who she wants to talk to, since social interactions are tiring to her.
In conclusion, Irene did not talk the adult BECAUSE she did not want to talk to them. It was as simple as that. She did not trust them or like them at all and she believed that she had the right to not respond to their talking, haha