I've come across this conversation exchange in the transcript of a textbook:
(the speakers are discussing that a friend of theirs has been dismissed because he was sending personal emails from the company's computer)
Laura: That's a bit unfair, isn't it?
Dom: You say that, but actually what happened to him was he visited ...
I think I can guess what the meaning of "you say that" is: "you say that because you don't know the whole truth", but I'd never heard that phrase. I'm looking it up in different dictionaries but I can't find it.
Is it an usual phrase to indicate that somebody's guess is wrong?