It's just not possible to precisely grade any coin using the grade definitions available.
OK, maybe it is but only with brand new therefore modern coins fresh from the mint. It should be possible to say if a new coin is a 70 or not.
However, once you start looking at older coins with some wear, it becomes impossible. All coins are different and will show wear and faults in different places and to varying extents.
Even if you get several graders to look at each coin, you still have to realise it's human beings giving their opinions by comparing the coins to the defined standards. It's not possible to use some kind of scanning technology etc.
Once you accept this fact, it follows that sometimes you will receive grades less than you expect, sometimes the opposite. I have had both.
The question is what do you do?
If it's better, do you just smile and rub your hands together with glee? Or do you send it to be re-graded?
Ha I think I know the answer.
Once you accept that there will be "errors" made by the TPG's, what's wrong with re-submitting hoping to get the right result next time?