Newcomers usually misunderstand the grading scales. To get it right, you should look at it as if the grading scale has three ‘buckets’. The first is for circulated coins, the second for About Uncirculated or AU coins, and the third one is for Uncirculated Coins, or Mint State or MS coins. Know that the MS scale is completely different from the AU scale. Many think MS scale succeeds AU but that is not the case. MS is actually a mini-scale and starts with MS-60, which is the ‘basal state’. This one may be ugly with no luster marks but it is definitely UNCIRCULATED. AU-58 on the other hand has luster and is attractive but it is about uncirculated. To make the confusion clear, know that they are of different ‘buckets’. That is the reason why the lustrous AU coin comes beneath the ugly MS coin.
The AU min-scale begins with 50 and ends with 59. The AU-50 coin may have scuffmarks even if it has never been circulated. That is why it has been given that place. It has been handled at least a little and is NOT in Mint State. This is why you can see that the ugly one comes in AU-50 and the lustrous one comes in AU-58. This explanation is as simple as it can get and will hopefully clear the confusion about why the scale keeps jumping from appealing to ugly and then back to appealing.