Ok. So I'll add a bit more to this.
I asked Carl if he's read the HC books and he claims he read the first one and admits he doesn't feel its plagiarism.
However, this is his feeling.
In 2010, he made the editors desk for his book. HC gave him a favorable review and said the book had potential. But they didn't offer him a contract.
Two years later, the book comes out through HC.
Carl believes they stole the idea after giving him an editorial review.
There's a good thread going on Authonomy right now, although it's mostly paranoia, the HC admin comes in on the 14th page and tries not to stroke the fire.
http://authonomy.com/forums/threads/135919/are-our-books-really-safe-here-/?pagenumber=1
I still say it's coincidence. I seriously doubt the poor employees dealing with the editors desk each month have any say in how the chapter books are created. I'll also add that Authonomy is part of the British HC and not the American branch.