Why does the Windows 8 sign-in animation slide upward?

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.^{[citation needed]}

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.