I really think it's more the granularity issue than anything, although I'm sure there is at least some selection bias.
Most of the readers I know and have known, read at least a book a week, usually more. The average American watches 2.8 hours of TV per day (
source), and it's reasonable to expect that a person who lists reading as their primary leisure activity would devote a similar amount of time to it. That's just about 20 hours a week, which according to the 10 hour audiobook scale, would easily translate to 2 books a week.
If you use that as a baseline, you get the average "reader" at around 100 books a year, which is almost twice the rate of the highest band on this survey. A better survey for this site would have far less granularity on the lower end, and extend further up. I think the average here is probably in the 50-150 range, and that's just not something this survey's designed to measure.