I'm still undecided on this issue, but I would say that maybe because she's been thinking about him her whole life she thinks that she knows him.

And I'm trying to remember - has the Doctor mentioned other companions before? I mean, other than when saying he can't be alone for too long because he starts talking to himself? I can visualise him saying something like 'I have people who travel with me' but I'm not sure if he actually did or I invented it. (Ha, I'm getting as bad as Amy!)