I think it's because Ruby has no emotional connection with Dean. Her first though is, If he finds out, it will ruin EVERYTHING, not to mention he will kill me. And she has time through that whole little fight scene and the brother-hugging to come up with something to say.

As for Sam, he's thinking like a guilty person, because we know he's consumed with guilt even as he chooses to suck down the demon blood. He CANNOT let Dean find out about that, since, as he says, it was practically Dean's dying wish that he not use his demon powers. So Ruby's little hint about them being a couple, when she KNOWS they aren't, is enough to remind Sam he needs to lie, and it gives him an easy out to get rid of her while he decides what to do.