That is called an inside joke.
Yeah, it was just done to show that Buffy was becoming a bit isolated and felt a bit apart from the rest of the group. They just had an inside joke that Buffy (and we, the audience, by extension) didn't get. Because the goal is for us to feel sympathetic to Buffy in that situation, we don't get the joke either.
I second the inside joke post.
But did anyone else think that this scene was super funny? 😀