Also they finished the work in a brisk sprint-like pace towards the end, knocking out the pages in record time. I think that was more indicative of the actual effort required to finish the work, not all the hours they billed. There is a perverse incentive when the client has a retainer to bill hours and make up excuses to line their pockets. They finessed the author and preyed on his naivety
They started work on and subsequently billed for something that was explicitly out of scope.
My guess is that the founder of the agency knew early on that he could push this client around to extract $.