if someone who appears to you to be outgoing, tells you that they are shy, believe them. What you are seeing is their performance face, their coping mechanisms. It may come over as arrogance, be expressed as sarcasm, be over ebullient, or talking too much. It may exhaust them so much that they can't think too straight about the reality of a situation while they are "performing". It may not be a good coping mechanism. But it is not proof that they have lied about their shyness.
And finally: there is a hierarchy in inter-actions which means someone who is well known to a group is facing a room full of strangers. You may see them as a celebrity. They see themselves as someone desperate for a familiar face. I have lost count of the number of "arrogant" authors, editors and academics I have come across who turn out to be really lovely people when not surrounded by thirty people they have never met before. In a way, it is a privilege to be relatively unknown and to have your nerves and shyness accepted as nerves and shyness.
To which I would add, the same applies on line.