At that time, publishers made a killing on frontlist e-book sales as compared to
frontlist hardcover sales — at the author's expense — because, as compared to today, the price of e-books was relatively high.
Not exact matches
So the e-books include almost every title that is selling, while the
hardcover sales are divided among a much smaller set of
frontlist titles.
(You also have to factor whether a discounted
frontlist ebook is going to further cannibalize any
hardcover sales, which still yield bigger credits to the author.)