I think that at the time, the agency upsold the author and they went along with it; after that, sunk cost became a factor. I do believe this is what most agencies will try, upselling I mean.

That said, clients can also be problematic; I recall one where the client paid a fixed amount per two-week sprint, but then really started to stretch out how much work was done within that sprint by basically not signing off on tasks and reporting new features to be added as bugs. It's another case I think of a party not being firm enough about scope.