Please look at the pictures showing the Centimeter Scale and the Inch Scale.
Now say your ring size measures 2.7 centimeters. See how easy it is to read.
The equivalent of this in inches is exactly 1.0767
Unfortunately, the Inch scale does not have these divisions, it only has 1/8, and maybe 1/16 divisions and you can guess at the 1/32 division.
For a 2.7 centimeter size, the equivalent inch size is 1 3/32 inch, which is difficult to read, so most people will use the nearest readable division which is 1/8 or 1/16, giving a wrong size.
This is why we use the centimeter scale.

How to measure your size in Centimeters