It seems you never had to work with that specific kind of people who write the tasks (no matter the size), but : 1°) don't know exactly what they want themselv

It seems you never had to work with that specific kind of people who write the tasks (no matter the size), but : 1°) don't know exactly what they want themselves, so the task is missing key points you discover while coding the task, or 2°) are writing tasks for someone else and the communication is not good so some info is not clear or missing, or 3°) the task is written too soon ("just in case") and needs external information to be complete, or 4°) change their mind while you already begun to code