'Copies of books' is not what is meant by 'output.' That is what is meant by 'sales.'

Example:

99 books with 1 copy ordered, 1 book with 198 copies ordered 

-> 1% of output is 66% of sales.

edit: corrected math, it's actually a crazier ratio than I thought at first.

'Copies of books' is not what is meant by 'output.' That is what is meant by 'sales.'

Example:

99 books with 1 copy ordered, 1 book with 66 copies ordered 

-> 1% of output is 66% of sales.