It's easy and yeah, it has been discussed before. You plan, you do, you test and replay what you did, you see things don't work and have to redo them, so you add another task to show things didn't go as planned and you'll need to work some more, creating different artwork, dialogues or whatever. At some point, all the planned content is finally done and then, when you replay what you have (your 'alpha' version), you add editing tasks to show you aren't happy with the results and you need to keep fixing, improving and redoing some things. As simple as that. You call it moving the goalposts, I'd say is being transparent with your supporters and telling them the update will take longer as soon as you know it will.