Well, i had an old crap 17" no-brand LCD monitor do the same thing. turns out, it was excess pressure against the panel from the backlight. (bulging plastic).

But sometimes i guess the panel could get an air pocket? and have the same effect? not sure.

But its the same mysteries that ive seen some LCD panels that have gotten cracked, and STILL worked with no signs of liquid crystal or electrical damage. Because usually the crystals escape and/or electrical connections are broken.