Well, local dimming backlit LED LCD's are setup in blocks across the screen, meaning random LED's cant just shut off, one of the blocks (cluster of LED's) has to either be off, dimmed or on. When you have a bright area, say a face int he dark, that touches into a block that should be off because the area around the face is actually dark, it has to light to show part of the face. This affects the darkness off the rest of that block. If you moved a small white ball across an entire black screen, you'd see the blocks turn on and off, and sometimes this bugs folks.