If you've ever wondered why contrast ratios claims of LCD monitors can swing so wildly from product to product, here's the explanation:
There are "dynamic contrast ratios" and then there are "static contrast ratios". Static contrast ratios is defined by the ratio of the brightest part of an LCD screen to the darkest part of an LCD screen that can be simultaneously displayed on the screen. Dynamic contrast ratios are measured by the darkest dark from one image to the brightest bright from another image being displayed at different times.
This is achieved by dynamically altering the brightness of the entire screen or darken the entire screen when scenes are mostly bright or are mostly dark. This increases the "blackness" of the screen during dark scenes and it increases the brightness of the lighter scenes.
This effectively allows LCD makers to claim a larger dynamic contrast ratio of "3000:1" as oppose to a static contrast ratio of "800:1". While this technique can definitely improve the video quality for some mostly bright scenes or mostly dark scenes, you can't actually get that level of contrast on the screen and the actual contrast ratio is not altered.
So when you see these inflated contrast ratio scores, you're not being lied to but the numbers are confusing. Having an LCD that dynamically shifts brightness is a desirable feature but it isn't a substitute for true contrast ratios.