Saying they are very generous is not accurate. The two TPGs are fairly close in the way they grade (in fact NGC is more strict on some designations, like FBL for Franklins). Some NGC will downgrade at PCGS but some PCGS will downgrade at NGC. Some will upgrade at each service. And plenty would get the same grade. A big part of the reason that PCGS coins sell for more is marketing. Enough dealers and collectors have pushed them to the top. If a coin was graded X at NGC and then crosses at the same grade X at PCGS, the price increase can’t be explained by generous standards considering both companies graded the coin the same. Another interesting thing is that when one looks at world coins, NGC coins tend to do better. And plenty of world collectors say PCGS can be generous with grading (as seen in a recent thread on here).