system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is the coin grading system a scale of 60-70? That seems like a rather odd range.
gpt
The scale was originally proposed by William H Sheldon.  Since no such scale was widely in use at the time he had a very wide amount of leeway in choosing what the scale would look like.  Ideally the scale should have enough different grades that you could have a good idea of how worn a coin is, but few enough grades that there wouldn't be a bunch of disagreement between experts as to what the grade of a coin actually is.

That sweet spot is not necessarily aligned with a nice round number that you might hope for, like 50 or 100.  50 was too coarse of a scale for Sheldon, apparently, and 100 was too fine, so he settled on 70.  Since he was the first to propose a scale that gained traction (originally just for large cents, but it can be extended to other coins) it became the industry standard, with a few minor modifications over the years.

Note that the scale doesn't just run from 60-70, as your title may suggest.  The scale runs 1-70.  It's common to see coins only listed between 60 and 70 as those are the highest grades and are the range where you could describe a coin as being in "mint condition."  Also, proof coins (struck more slowly and carefully with polished dies) typically land in this range.  Most coins are much worse off, but if you have a poor condition coin like that then it's not as common to go get it graded.  A coin from the last decade in MS-68 condition may fetch a premium, but the same coin in AU-50 condition is just worth its face value, so it's silly to pay to have it professionally graded.