Sheldon Scale

My guess..and I emphasize guess...
When you are speaking to the sub MS grades, the reason for the bigger jumps in numbers is that the difference in value between say F-12 and F-14 would be so small that it isn't even worth worrying about. It isn't until you move up 5 points in grade do you see much change in value. On the other hand, when you get into the MS grades, particularly high MS, small changes make large increases in value. The difference between an MS66 and MS67 can double the value of many coins.

"small changes make large increases in value"

I would change this to "in perceived value."
At this level of preservation, collectors who 'chase the highest number' are the ones
who pay dearly for one number higher up.
We ordinary collectors who chase nice, decent circulated examples are perfectly happy
with the current Sheldon System. That is, if we care about the Sheldon System at all.
No number can replace the traditional word descriptions. At least, among old time collectors.
There are a few of us who cannot see the difference between an MS-64 and MS-65.
So, we do not bother with them.
But, we same collectors can easily distinguish between a VF and an XF coin.
And we buy accordingly.

Oh, and just guess who are sitting on the nice coins - the old time collectors!

My educated guess.

While the numbers in the Sheldon scale 1 through 59 are continuous, it is impractical to have 59 grades for circulated coins. Can you imagine trying to distinguish the difference in wear between, say a VF30 and VF31. Impossible!!

The ANA came up with specific numbers to define grades with each higher grade getting a higher number and a definition or description associated with the grade.

For example AU 55, Choice about uncirculated-55
AU 58, Very choice about uncirculated-58

So, we have 19 steps (grades) for circulated coins (Poor-1 through AU 58) and 11 steps (grades) for uncirculated coins (MS 60 through MS 70)

It makes a lot more sense when you understand how Sheldons Pricing system came about. When Sheldon was writing Early American Cents in 1949 he had noticed that prices had remained fairly stable for the previous quarter century and Sheldon noticed that for any given 1794 cent variety the price of a coin in Fair, was twice that of the same variety in Poor or Basal State. Furthermore a coin in Good tended to sell for 4 times the price of a Poor and a Fine coin for three time the price of a Good. A VF sold for five times the Good price and a VG sold for twice the Good price. An XF sold for 5 times the price of a VG. A AU coin for 50 times, MS coin for 60 times, a Gem MS for 65 and Perfect MS for 70 times as much as a Poor. So if a Poor 1794 cent sold for $1 A Fair sold for $3 a Good for 4, VG for 8, Fine for 12, VF for 20, XF for 40, AU for 50, MS for 60, 65, or 70. Sheldon though he had discovered some natural law that controlled the ratio of cent prices. The reason we now use all 11 pints from 60 to 70 comes from greed and trying to justify charging more for your coin. When the ANA grading guide was published it only had three MS grades 60, 65, and 70 and 70 was considered to be more of a theoretical grade. Befor long though dealers were trying to justify a higher price because "Well it isn't good enough to be a 65 but it has to be better than a 60." And the same excuse for why their 65 coin had to be worth more. So eventually the ANA caved and added 63 and 67 to the grading scale. A year or two later they were back at it "Well it's not a 65 but it has to be better than a 63." and the industry unofficially adopted MS-64. The ANA was pressured to accept the 64 grade and instead just bowed to what they knew would inevitably happen and added 61, 62, 64, 66, 68,and 69. PCGS though was the first TPG to actually officially adopt all 11 grades. And now today they have added to + grade which means for all practical purpose a .5 grade giving 21 grades of MS. (When the + grades were announce they also mentioned that in house they now use all the decimal point grades or 101 MS grades. I would not be at all surprised if we don't start seeing them someday.)

The only thing I would change is to add a grade 11...

If you go to enough coin shows you would actually see almost every grade imaginable. And too, there are many that still use only G, F, Unc and Proof. Even in the Red Book there are many pages using only a G, F, VF, Au, Unc. Some dealers tell me it is easier to sell a coin graded as F to older people since that is what they grew up with. An F-12 would not mean to much to many.
And too, some dealers put a EF-40 r XF for for a Extra Fine. Then too so many use BU, ABU, instead of a MS grade. Naturally not so many variations with slabbed coins but nany dealers like to keep it simpler. A BU is so much easier to sell than trying to explain why a coin is a MS-63 or -65 and not just a BU or Unc. And too try to explain where a Unc leaves off and a MS begins to people. I've seen AU-58, -59 and even -60. So far no MS-47's though.
We have gone from simplicity to extreame complications in grading so why not make it worse. Lets change to a 100 point system, with decimals. That would keep in similarity to the metric system. Of course we would then meed additional letters for the grades too.
Example would be a AEF-84.55 for About Extra Fine + or -.