Whopping $4000 difference in selling value!!!!

I was browsing through the ebay for the 20th Anniversary 3-coin Gold Eagle set (mine has not arrived yet from the US Mint, and I was curious how much the set was selling on ebay). I was surprised to see how much they were selling for on the auction blocks, but more suprising was the price difference between PCGS graded set and NGC graded set. The PCGS graded set of PF/MS 70 were selling over $9000 while the NGC graded set of PF/MS 70 was selling little over $5000. I know that the 3-coin anniversary sets were sold out in a record time, but $4000 price difference between PCGS graded set and NGC graded set? Hmmm... I wonder if the paper money graded by PCGS are also valued higher than PMG.