Why would a girl be so chatty online and not too talkative in real life?

Me and this girl talk everyday and she keeps going on talking online but in real life, she's a bit quiet and takes a while to get into the gear for talking and laughs a lot later. She often waits for me to come and doesn't talk to others much. Any reason why?
Why would a girl be so chatty online and not too talkative in real life?
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