Background: Expressing Film Speed on a Logarithmic Scale
In 1960, the ASA (American Standards Association, renamed the American National Standards Institute, or ANSI, in 1969) published a standard:

ASA PH2.5-1960. American Standard Method for Determining Speed of Photographic Negative Materials (Monochrome, Continuous Tone). New York: United States of America Standards Institute.

This standard defined a method for determining the speed of continuous-tone monochrome photographic negative materials, in which a method for expressing film speed using logarithmic values was proposed, namely:
$$S_v = \log_2 (N S_x) \tag{1}$$ where $S_x$ is the arithmetic scale of ASA speed; $S_v$ is the logarithmic scale of ASA speed; and N = 0.3.

For example, for $S_x$ = 100, $S_v$ = log2(0.3 x 100) = 4.9 ≈ 5, meaning ASA 100 = ASA 5°.

Background: Calculating Camera Exposure on a Logarithmic Scale
To simplify the calculation of exposure in photography, the ASA published another standard for general-purpose photographic exposure meters in 1961:

ASA PH2.12-1961. American Standard, General-Purpose Photographic Exposure Meters (photoelectric type). New York: American Standards Association. Superseded by ANSI PH3.49-1971.

This standard integrated the relationship between scene luminance, film speed, exposure time, and lens f-number into a single equation, namely:
$$E_v = A_v + T_v = B_v + S_v \tag{2}$$ where:
Ev (Exposure Value) denotes the exposure value;
$A_v$ = log2(A^2), where A is the f-number of the lens;
$T_v$ = log2(1 / T), where T is the exposure time (in seconds);
$B_v$ = log2(B / KN), where B is the average scene luminance (in the imperial unit of foot-lamberts or fL), K is the calibration constant for reflected-light meters (defined by the meter manufacturer and generally derived from extensive experiments), and the ASA PH2.12 standard recommends a K value of 3.3333;
N and $S_v$ are defined in ASA PH2.5-1960.

Rearranging Equation (2) yields:
$$2^{E_v} = \frac{A^2}{T} = \frac{B S_x}{K} \tag{3}$$

From Equation (3), it further follows that
$$B = \frac{2^{E_v} \cdot K}{S_x} \quad (\textrm{fL}) \tag{4}$$

For a Lambertian surface, considering the unit conversion between the imperial and metric systems for luminance,
$$1 \space\textrm{fL} = \frac{1}{\pi} \textrm{cd/ ft}^2 = \frac{1}{\pi \cdot 0.3048^2} \textrm{cd/ m}^2 = 3.42625909964 \space \textrm{cd/ m}^2 \tag{5}$$

Equation (4) can then be written as
$$B = \frac{2^{E_v} \cdot K \cdot 3.42625909964}{S_x} \quad = \frac{2^{E_v} \cdot K_m}{S_x} (\textrm{cd/m}^2)\tag{6}$$

The difference between Equation (6) and Equation (4) can be equivalently viewed as a change in the K value, namely, the metric unit Km = 3.42625909964K ≈ 11.42. ISO 2720:1974, which is similar to the ASA PH2.12 standard, uses metric units and recommends a Km value range of 10.6 to 13.4, while common exposure meter manufacturers define Km values ranging from 10.6 to 14.0. The metric equivalent Km value of 11.42 for ASA PH2.12 also falls within this range.