It's just a way to reuse the footage and to throw the viewers a red herring.I'm thinking that Roy has to have killed Sarah, possibly under some kind of mind control by Ra's or something. There's no way he could have dreamt about the exact dialogue that took place on the roof unless he was there. If it wasn't him, then it's a ridiculous and blatant oversight by the writers.
Yeah, it's a suspension of disbelief thing - same reason when anybody on a TV show has a flashback you see it from 'our' perspective rather than theirs. TV shorthand.