The number of professional bloggers is based upon that technorati survey (based upon 1300 technorati users) where 2% of users said that blogging was a primary source of income for them. So basically 26 technorati users have been scaled up to half-a-million pro-bloggers.
the $75K average is skewed by a handful of outlier successes, but the great majority of bloggers who get 100,000 uniques/month earn more like $22,000. Here, the median is far more relevant than the average