I'm not sure why this is, but it's been my experience that some people do not consider it "lying" when they tell someone else something because they think it's what the other person wants to hear, as opposed to telling someone the truth even when it may not be the answer they would like to hear. It's their way of following the path of least resistance. Your couple sounds like this kind of modus has been working for them, but it doesn't seem to be working for you. You're gonna have to get them both together in the same room and say "Look, we need to talk about some things."
Remember - you can love someone, and be "in love" with them, and still not make good partners for each other.