I've been snubbed, and think I've accidentally snubbed others as well.

Close to home I don't see many cyclists out, so I wave to everyone. Most usually wave back. But in busier cycling areas, I usually only wave if I see someone waving to me...otherwise I'd be waving constantly for an hour or so.

One time I was riding along a straight road solo. Was concentrating on the road, thinking about what was ahead of me, daydreaming, etc. Suddenly I saw a guy going the other direction waving wildly across the road. As soon as I could I waved back. But I felt pretty bad.

Like the OP, I've waved to some people, said, "morning," etc...and had them look at me with a blank stare. It doesn't even phase me anymore. Maybe they're deep in thought, maybe they're visiting from another country and don't know our customs/language...or maybe they just don't do that. Who knows. It has happened no matter how much everything I'm wearing matches or doesn't.