In every project I work on, there is an initial discussion on scope. In the almost decade of working on tech projects, I have never had a client who had a perfect scope in mind. There is a process of discovery where you delve into the goals, needs, desires of the client. Even when that process is smooth and everything is documented to a tee, as the project moves forward the client begins to see how things can be made better here or there.

A good example might be discovering that sorting a list by last name might achieve one business requirement, that requirement could be enhanced by allowing sorting by date.