The grading of coins is more of an art than a science. There are several coin grading services, however, they all lack consistency.
ACGSlabs strives for consistency. While no two coins are exactly the same, two coins, both graded MS65 should be as close as possible to having the same number of visible and sub-visible defects. If not the same number of defects, the sum of the defects' size should be comparable.
Here is the ACGSlabs standard for testing the veracity of a grading company; for a given grading company, select a coin by denomination, date and mint that you like. Select a grade for that coin. Now search your favorite source for buying coins, and find at least 5 coins of the same denomination, date, mint and grading company. What is your comfort level that all of these coins deserve the same grade? If your comfort level is low, you should not buy coins graded by that company.
Most grading companies pride themselves on having the "highest " grading standard. Some boast of having the most demanding standards. Some of the oldest companies are known for changing their standards. Their early grading efforts regrade at least one or two grades higher if regraded. We believe that the standard used is irrelevant if all coins given a specific grade are not worthy of the SAME grade.