Bill Gates walks into a bar, suddenly the average wealth of people in the bar goes from thousands to millions. And also now everyone in the bar except Bill has below average wealth. Similarly, contributors to user generated websites like Wikipedia are almost all below average. There are a few people who contribute a ton, and a whole lot of people who contribute very little. This is why it is often very hard for people to conceptualize how these user generated sites work- there is no “average user” to imagine yourself as.
(Basically paraphrasing from Clay Shirky’s book Here Comes Everybody)