Of course it's true that authors make wildly different royalty rates. Does anyone here really believe Stephen King makes the same royalty rates per book as someone like Uwem Akpan?

It's simple market economics. Stephen King will sell far more copies which means publishers will make far more money off his books. So he gets a significantly higher cut of the per book profit (aka royalties) because publishers will still end up making more money off his book than they will of a Uwem Akpan book.

Put it this way, if Mr. King's publisher didn't pay him a higher royalty fee don't you think another publisher would be willing to based on the large amount of money at stake? So again we're back to basic economics.