question about luma*2

[qihaoliu0]

Hi!
about RCAS, why it is calculated as L = B*0.5 + (R*0.5) + G? I searched Luma at wiki。
B's coefficient seems is always <= 1.2, why using 0.5 as B's coefficents in code?

  // Luma times 2.
  AF1 bL=bB*AF1_(0.5)+(bR*AF1_(0.5)+bG);
  AF1 dL=dB*AF1_(0.5)+(dR*AF1_(0.5)+dG);

I'm very confused about this , cloud you please teach me?
Thanks so much!