I'm fairly sure that I've seen these PB's on here before. My theory is that the turnbull mounts are relevant to the foreshortened wall. If Vickers were intended to be mounted in the embrasures and the internal wall was full height, it might hinder operation of the weapon ie space to the rear being insufficient? The required space to the rear of the embrasure for a Vickers on a Turnbull is 4' 9" so a simple measurement on site would reveal whether this is a factor or not.
They may have considered that as long as the wall reached to below the level of the embrasure it could prevent the ricochet effect as much as if the wall went right to the floor. The gun crews would have the best of both worlds ie - room to move plus ricochet protection. Just a theory.
PS: There's a nice accessible Pickett Hamilton at Worthy Down. Hydealfred reported on it here a while back and was good enough to take me for a looksee. Fantastic.