Quasi-chemical model in thermodynamics

[asdf1]

For the quasi-chemical model in thermodynamics,
there's a part where there's an equation,
E= E(AB) - 0.5[E(AA)+E(BB)]
where E(AB)= bonding of AB
E(AA)= bonding of AA
E(BB)= bonding of BB

but why do you need to multiply 0.5 in the equation?

 

[oydvoid]

E is the difference between AB bond energy and the average of the AA & BB bond energies taken into account.