Book behind xp level on enchanting table flashes

I am not sure if this is a bug or secret Easter egg.
When have the enchanting table open with nothing on the left side that can be selected (no greyed out items either) and there are xp orbs around, the book behind the xp level flashes like an xp orb.

Also at night the book apears to dim and brighten.

My set up

Flashing


Not flashing