Posted By:
Eric BrehmA numerical grade is only a first approximation to the desirability of a card, condition-wise. Third party graders have to apply a standard that can't possibly match all collectors' feelings regarding the effect of different condition attributes on desirability, and sometimes they just make mistakes. As long as collectors keep that in mind, and always buy the card, not the holder, they should be fine.
For my T206's, centering is more important to me than sharp corners, so I buy many EX 5 examples and reject many NM 7's that I see for sale. To echo what Steve said, this is win-win for me since I get the card I prefer, and usually save some money in the bargain. The grading standard isn't wrong to assign a '7' to a card I don't like; it just doesn't quite fit my tastes in this instance, so I have to take it with a grain of salt.
On balance I think third party grading is good for the hobby. Graders (in theory at least) provide a starting point for condition assessment by screening out fake and altered cards, identifying subtle defects that are difficult to detect, measuring the centering of the image, and so on. The rest is up to collectors to figure out.