No comedy intended Fisty....
..but as you pointed out... it is mostly true... the one you promote the most/best will be your best..
GG doesnt put his "sheer numbers" in his stats, but I would imagine you could find some correlation between traffic sent and conversion success.
I think it is a combination of marketing and niche appeal.. but knowing the answer to that one will be different for each webmaster... a bit of "suck it and see" is needed.
DD