If you read the comments one person says they put a translucent dome over the face, but you can still see the skull face under certain lighting. Hence this picture.
I thought it was because you had to apply so much force that the whole top of the bag will break open. Then you lose half your chips in the explosion and also feel more compulsed to eat more now that the bag can't be resealed. Then you will go to the store sooner to buy more. ;)
http://i327.photobucket.com/albums/k469/whatisbam/aliens1.jpg