That's definitely a part of it. A lot of players when they join the site for the first time start several online games, and then never log back on again.
It's also just due to the fact that you can't really compare ratings across different areas. My Tactics Trainer and Chess Mentor ratings are in the 1800s, but I'm nowhere near that rating in other areas because they are on totally different scales.
Why are people's online chess scores, on average, higher than their other scores? My conjecture is that people tend to time out often..