Well it certainly is a pity if not being greeted is taken to mean that the other person feels superior. It could be the other way around - the other person could be struggling with his or her bike, or tired, or lost, or just deep in thought - I spend lots of my commute mulling stuff over. Or just concentrating. I probably look quite fierce when I'm working hard and deep inside my own head, and not expecting to meet anyone I know. I'd probably get around to greeting somebody back 500 metres after they'd passed...