since u r using std cost on parent, and avg on material. The variance is obvious since ur parent cost will be different from the avg add-up from material.
The difference will be posted into variance.
WIP is just a temp space to allow u describe the status when parent is not done, and and material is issued.
So if u think ur transaction is done in a single step, material disappear from inventory, and finish good appear in inventory at the same time.
std cost of parent - sum of material's avg = variance.
This is something u can not avoid since u r using different cost method.
Frank