He also benefits from the inclusion of Harry Potter and the Goblet of Fire, which came out before his quote skyrocketed.

FORBES: Magazine Article

They were series opener Harry Potter and the Philosopher's Stone in 2001, fourth instalment Harry Potter and the Goblet of Fire in 2005 and the fifth entry, Harry Potter and the Order of the Phoenix, in 2007.

BBC: Harry Potter film breaks UK box office records

After Wilson's Tracy Beaker, JK Rowling's Harry Potter and the Prisoner Of Azkaban was the most borrowed title, followed by the Potter titles Chamber Of Secrets and Goblet Of Fire.

BBC: Jacqueline Wilson most loaned author of the decade

What happened was, so Goblet of Fire, I was really in quite a state by the time that book was finished, and I mean at that point I really did feel a lot of things came together with Goblet of Fire.

BBC: JK Rowling

So when I finished Goblet of Fire, I said to - there were only two publishers who had bought the next book - and I said to both of them, I want to repay my advance.

BBC: JK Rowling