Buy the Card, Not the Case
Grading sports cards is a very subjective process. The major grading companies do a great job overall but just like any other business, they can experience turn-over within their grading staff, can have an "off" day, or just flat-out have an opinion that the majority of the hobby will not agree with. Modern cards are fairly straight-forward and most graders will agree on a card's overall condition. However, opinions on a vintage cards' grade can vary dramatically from grader-to-grader. A good rule of thumb is to buy the card, not the case. As you can see in the pictures below of the two Jackie Robinson rookies, both cards have the same exact grade. However, one of them is in a completely different ball park when it comes to quality and the overall condition of the card. The higher-end card most likely has a surface wrinkle or light crease but that doesn't change the fact that every attribute of the card exceeds the technical grade assigned to the card. The difference of opinions can work in your favor. In some cases, you can get a 8 that is in a 6 case or a 4.5-5 in a 2 case.