I hear that a lot and it appears to be true. I want to stress that the following are merely my opinions...
While it’s counterintuitive to me, I believe that the ultra rarities are graded more liberally than coins of lesser value and importance. The major grading companies want the numismatic prizes in their holders and play a game of who’s willing to grade them higher in order to achieve that.
If I remember correctly, one particularly conspicuous and important example is the King of Siam 1834 Proof set. I believe that over the years, a number of the coins in the set have migrated between PCGS and NGC and have received grade increases of one or more points. Never mind, that it’s not like they started out with conservative grades, either. There are other examples of significant grade increases for1804 Dollars,1913 Liberty Nickels and so on.
Regarding ranking of the ultra rarities - that’s a lot easier to do (with grades) when there are only between 5 and15 known examples of a particular coin, than when there are dozens, hundreds or thousands. After all, in a perfect world, wouldn’t all coins be ranked correctly by accurate grades?