Equivalent shear modulus in LIGGGHTS

Hello eveyone, recently, when i use LIGGGHT and checking its parameter, i saw a strange thing when calculating Geff. that is:
In the function createGeff (global_properties.cpp). Effective shear modulus is calculated follow:
https://www.cfdem.com/system/files/screenshot_2024-10-01_153349_1.png
Howerver, as contact mechanics theory, equation 8.20: For a contact between two elastic bodies with the elastic constants
G1 , G2 , ν1 , and ν2 is:
https://www.cfdem.com/system/files/4_2.png
with Shear-modulus is: G = E/[2(1+ν)]; Then Geff should be:
https://www.cfdem.com/system/files/3_4.png

Could any one can explain it for me? why are they use this equation?
https://www.cfdem.com/system/files/screenshot_2024-10-01_153349_1.png