I think the other commenter nailed it: in text-only chat, you end up projecting your own ideals upon the other person.
Human communication is full of nonverbal cues which do not translate at all over text. In absence of those nonverbal cues which our brains are (usually) wired to pick up on, we end up mining the text for semiotic constructs which conform to our ideals of the other person, rather than the reality of the other person.
This is done subconsciously, and doesn't become obvious until you meet the person in real life and they are completely different from what you imagined.