The Rev A AlBook 15" models had the same problem due to pressure on certain sections of the LCD during shipping.
I would hazard a guess that at some time the iMac had undue pressure on the LCD. Due to the position of the mounting neck, it developed that white spot.
I would hazard a guess that at some time the iMac had undue pressure on the LCD. Due to the position of the mounting neck, it developed that white spot.