iMac question

Could be the magnets holding the screen, since now the LED panel and glass are fused together since they introduced the ultra-slim design in late 2012.

 

I don't own an iMac, but there are iMacs all over the floor here at work; we have models going from 2008 until 2013, the 2012 and 2013 ultra-slim models all show those black dents/shadows..

 

Depending upon the angle you're looking at the screen, you could be seeing the LED back lighting. On the 2012-2013 models there is a row of LED's along the top of the screen that provides the lighting. If you view the screen from down low (imaging you're crouched down at the foot of your desk, looking up at an extreme angle to the top of the display). This is also why some panels show more of a yellow cast toward the bottom of the screen.