Posing in this context means laying out (using code) the images required for a particular scene, which may require overlaying / overlapping some of those images.
Because they need the (new) images to be drawn before they can pose them in a scene, and before they can implement the code required to show the relevant dialogue and to implement the interactive elements of the scene.
Sometimes extra tasks/activities get added to the List of Things to do for the next release, this causes the total number of things to be done to go up, which in turn causes the percentage of completed things to go down.