And the manufacturers often exaggerate, or offer numbers derived from unrealistic conditions.
The only values you can really trust are those provided by reputable reviewers who have the equipment and the experience to test properly. Even then, you can only really compare numbers between tests done by the same site/reviewer, because different reviews that test in different ways may produce valid results that aren't strictly comparable. http://www.tftcentral.co.uk
does a good job, for example. However I don't believe they've reviewed the Samsung you're looking at, unfortunately.
Even an "eyeball test" in a store can be a bit misleading for things like contrast if the lighting conditions there don't resemble the surroundings where you will be using the screen. And, as Fox says, "dynamic contrast" can be a bit bogus regardless.