Long story short
There are two types of scanners: CCD and CIS. CCD scanners are able successfully scan items that are slightly raised from the scanning surface. CIS scanners only work if the scanned item is a purely flat object that can be rested directly on the scanning surface.
In the case of graded cards, the card itself is a few millimeters from the scanning glass because there the plastic from the holder is in between the card and the scanning glass. Accordingly, a CIS scanner will product a blurry image while a CCD scanner will ensure that the card inside the holder is perfectly focused.
Bottom line: if you're scanning graded cards, you need a CCD scanner. Usually these scanners are much thicker than CIS scanners, so that's one way to tell them apart. You should also be able to find technical information with the scanning element type, available online for your scanner of choice.
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