When I lived in a small town in NC, I noticed this exact same thing. I rode my fast road bike on my commute and I'd get nods/waves by the roadies I'd pass (they were out for evening rides - not commuting...hardly anyone commuted in that town but me). When I was riding my oddly set up touring bike with the rack and lights on my commute...the exact same people in the exact same location would snub me. Initially it annoyed me. Then I started to find it funny.

Now I ride in an area where there are just so many riders of so many types that waving/acknowledging every one would just be annoying. I do try to make a point of waving at all women on bikes (no matter the bike), so I guess that's my own 'little group' in my head.