The early design for the animation that occurs when you sign in was to take your logon wallpaper and open it up like a curtain.
This worked well for nature photos.
However, it is very common for users to put pictures of people on their logon wallpaper. The likelihood of putting a person on your logon wallpaper is strongly correlated to whether you have children.
It turns out that people don't react well when they see their children's heads cut in half.
That's why the sign-in animation is careful not to cut the picture into pieces.