The grading curve has dropped because the number available has dropped as well. Folks are more willing to accept a lot more wear on them.
That one wasn't "perfect" in operation nor appearance. It was definitely in need of restoration. Tin plate collectors were always the toughest train graders imo. I wouldn't have given it a four and it needed cleaning to have a chance get to three.
It is also a collectors guide, not an operators. Think of shelf queens being your only goal.
On the other end of the spectrum you have C10, you'll likely never see any of in any make, year, model.
I see the rest as C-9 are "exceptional", C8 scuff(s), C-7 scratch, c6 2 flaws, c5 wear, c4 heavy wear, c3 significant wear/damage, c2 poor to junk(savable) c1 parts junker(non savable)
As an operator if it rolls I'm quiet happy 
