this happens to me pretty often. i ride aggressively usually about 10-15 over and people still tend to pass me. i don't know if its the pipes, or if they get some sort of power trip from passing a motorcycle, but then again maybe they just drive aggressively like i do.
You're more likely to be passed on a bike because the passer knows you'll be glad to share your lane if he encounters oncoming traffic. It's common sense.
Maybe it's just here locally but I tend to pass guys on Harley's when I'm stuck in the cage because they can't seem to hold a steady speed better than +/- 5mph. The constant yo-yo-ing of their speed, and having to constantly hit the brakes/gas/brakes/gas so I'm not riding their tire is INCREDIBLY annoying!
When I'm on the bike I don't really get passed any more than when I'm in the car though. I drive them both kinda slow so I expect it.