Admit, you probably underestimate your traffic?
 

Whatever sites/niches you run, you cant resist also putting different kind of sponsors/niches on it even if its wrong niche for your site. In other words, you hope your visitors will maybe change their mind and go for "what is your best sponsor". ;) Now if you do that, its wrong, no wonder sales are down yet industry turnovers are increasing :)

I would call that traffic overestimation :) But yeah underestimation is good too, it depends on what angle you take :)

Or over stimulating your traffic ;)