If this was a single occurrence, I’d try not to read too much into it. Maybe they were discussing something private and got all weird when interrupted. Maybe the greeting was non-verbal and you missed the cue. If it’s the beginning of the day, they might not be all awake yet, I dunno.
But if it’s a pattern, or this ever happens and it bothers you, you can try to make the most of it. Imagine they wished you their fondest greetings in a Muppets style voice. It costs you nothing and you can’t change anyone else’s behaviour anyway, might as well do something to put a smile on your face.