I've heard it described as a "circle of focus". When you talk to someone and just seem to click, do you ever notice how newcomers seem almost intruding on the conversation, or only care about what the speaker is saying.
It's more for meeting new people, but imo the idea works with couples too. They obviously care an awful lot for eachother, what the other thinks, and the circle means they just want to focus on themselves.
It's not snubbing others per-say. If you try to break into their conversation, or pull them out of it, they should let you. But you'll be ruining good moments for them and they'd probably rather you didn't.
Alternatively, they were just matchmaking you and the other girl, like the previous poster said.