Via fjm:

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.

Oh yes absolutely.

And I'd add, from my experience (which may not be anyone else's): don't assume that someone who is outgoing and friendly on one occasion will always be outgoing and friendly. I often find social interaction exhausting: there are a few exceptions, but on the whole if I've spent a lot of time talking to or even being around people (especially people I don't know well) I become physically and mentally tired, and if I can't run away to a quiet private place I will run away inside my head, perhaps with the aid of a book or PDA, perhaps simply by withdrawing from the conversation.

You may apply this explanation to anything from my habitual playing of mindless games on my Palm to my preference for headphones on public transport to extended 'comfort break' absences (the bathroom was my only hope of solitude at Eastercon!) This is how I cope. It works for me, and it is never intended to be insulting or hurtful or rude: however, I do prioritise my own mental and emotional well-being.