Question about steel numbering system

I was doing some research on a steel supplier and came up with this and I have a question about one of the statements:
If the first digit is a one (1) in this designation it indicates a carbon steel. All carbon steels are in this group (1xxx) in both the SAE & AISI system.
The second digit of the series (sometimes but not always) indicates the concentration of the major element in percentiles (1 equals 1%).
The last two digits of the series indicate the carbon concentration to 0.01%.
For example: SAE 5130 is a chromium alloy steel containing about 1% of chromium and approximately 0.30% of carbon.
This statement:
"The last two digits of the series indicate the carbon concentration to 0.01%."
Does it refer to resolution or a ratio of some sorts and if it is a ratio then to what? Thanks

It's the percentage of a whole.

As in 4140, roughly 0.4% is carbon, 99.6% is all the rest (iron, chrome, moly, unobtanium, expensium, etc., etc.)

It makes a whole lot more sense than the European standards: i.e. DIN 1.7225 (which is 4140).

Thanks ChipSplitter