All grading methodologies have a tolerance of error built into them, but the systems used by the GIA and the EGL are highly reproducible and acceptable. Some less respected laboratories will sell grades. In other words, if a diamond dealer pays a higher price, he can get a higher grade. While this problem is not rampant in the industry by any means, a few laboratories are suspect to this kind of behavior. The EGL and the GIA have consistently been above reproach in this area.
Consumers have a tremendous advantage when buying certified diamonds.
Most consumers cannot grade diamonds. The truth is that some jewelers cannot grade diamonds accurately either. They either rely on their suppliers to grade the stone, which may or may not be accurate, or they tell the customer what the customer wants to hear. I think you get the picture. Diamonds with certificates give the consumer the ability to compare apples to apples, not apples to oranges.