It's probably because a lot of them are for a start, working in niche markets catering to their own demographic which are as a rule better paid than your average working stiff.

Also, it's generally more socially acceptable to be a fairy in less working class social circles... Hence more fairies coming out of the closet.

And last but not least, the selective handpicking of high-payed gay doctors, lawyers and whatnot as opposed to the mushroom pickers and manure shovelers the had on the straight side in order to dilute the results of the study.