I assume that the ballast carried in a glider will normally shift the C. of G. aft, causing the tailplane to provide lift, hence lowering the total weight carried by the wings, the actual "wing loading". This is not the same as A.U.W/wing area. Think of a strain gauge fitted at the wing root, but only think !
The A.U.W. could remain constant with a forward shifting ballast tank - but the wing loading would be higher, to compensate for the downward or negative lift required from the tail.
I am sure that there must be an abbreviation for this, but I have forgotten it.