Must be something they see using a 5x loupe. You might have 1 or even 2 of the slightest blemishes/lines/whatever that might not matter if we were talking about 67 vs. 68 or 68 vs. 69....but for 69 vs. 70, it matters.
I think the 95w is a quality coin but paying 5x for plastic is crazy when 99% of people can’t tell the difference between a 68-69-70. And 1/2 of the time there isn’t any real difference.
To further add the 2015w proof platinum eagle 1 oz. Coin has a mintage of 3,886 and I saw one for sale on e bay as low as $2,399 on an NGC proof 69 holder. This is the lowest mintage proof in the platinum series.