Contrast ratio is the ratio of the luminance of the lightest color, and the darkest one.
What you are really taking about is Dynamic Contrast.
If you can turn a part of the screen off, you can have a great contrast ratio. But there's a part of the movie they don't really tell you. There's not an LED per pixel, but groupos of LEDs. So if you have a group of LEDs off, and there's something bright near(more precisely, in) that group of LED you're not getting the real deal. Because something bright is almost shut off.
Wiki may explain it better than me:
http://en.wikipedia.org/wiki/Contrast_r ... rast_ratio