The bending moment M_b is larger in magnitude than M_a; consequently, the force resultant on the right side is larger than the force on the left side. It is this bending moment difference ΔM that creates the need for the 800 lb horizontal shear force to satisfy equilibrium.

In other words, no additional horizontal shear force would be developed in the beam if M_a and M_b were equal (that is, if ΔM = 0).