To be honest, I wouldn't be interested in the median author/designer. They're most likely putting little to no effort in to promoting their work. Discovering the level of interest you need to create to get near the top earners would be interesting.
Orman doesn't have very many themes on the site, but he does do a lot of off-site promotion on his blog (and perhaps elsewhere) and give away a lot of material as part of that. That could be a huge time-cost, but may be well worth it from his point of view given the earnings.
I may not have read it closely enough, but I couldn't work out how much of his earnings came from selling his own themes on the site, and how much came from the affiliate income he earned from selling other people's themes when the entry point to the site has included his affiliate code. That affiliate income could really add up if he's sending plenty of traffic their way.