About 10 years ago web-projects were mostly static and development process pretty simple. Now difference between web-applications and desktop applications disappears, functional complexity of web-projects grows up. It demands to new requirements. That’s usual now – the project is long-term , many of experts take part (non-experts as well). Everybody brings to the project backlog new ideas and goals, before the development stage as well as after. You see, goals and stories mutate and so the tasks.