NGC and PCGS grades are not the same. As shown in the graph below, for 1982 Pandas, PCGS clearly uses much tougher criteria than NGC. NGC graded 44% of 82 Pandas as MS69, but PCGS only allowed 16% to reach that level. In fact, you have to add PCGS MS67 (12%), MS68(16%) and MS69(16%) to equal NGC's MS69 rate. One could conclude from this that a PCGS MS67 coin could be equated with an NGC MS69 for rarity.
Dealers have noticed this too. They have sent 3 times as many coins through NGC. Could it be because they know NGC will give a higher grade?
Collectors beware. If you care about accurate and meaningful grading, stick with PCGS.