PCGS vs NGC grades

Being a "poor man" who enjoys the new Presidential Coin series I found myself standing by the sidelines and watching others snap up the gold spouse coins. At $500.00 per a commitment to purchase the entire series would be a $20,000.00 outlay over ten years (assuming gold stands still).

I went the frugal way and ordered my spousal, bronze metals from the US Mint. Generally disappointed is how I felt about the quality of the metals. Twice I sent back a metal which I thought must have been retrieved from the reject bin. Though sealed in original plastic wrap I found all of the metals stained, nicked or of shallow, mushy strike. Disappointing.

Imagine my delight when I discovered that both NGC and PCGS graded and encapsulated them. More to the point, I found metals of very high grade. At MS67, MS68 the First Ladies tend to be a brilliant red, with wondrous detail and I enjoy them now almost as much as I do my Mercury Dimes.

But,

why is it.......

that NGC has graded no First Ladies metal above MS68? If PCGS has a reputation for conservative grading then how is it that I had no problem finding Martha, Jefferson and Dolly metals at MS69

in PCGS slabs?

So, why is it?