Solid Difference make extra forms

Hey,

So I am trying to make a gs script that I can use to trim holes out of walls quickly for windows, doors, etc. based on the centerline of the opening. The final step involves using the 'solid difference' component trim the opening out. It worked fine, expect that it also recreates the original wall form (Image 1) and I only want the wall with opening (Image 2). It isn't a big deal deleting the redondant shape, but I really want to know if it is avoidable.

Does anyone know what causes this component to make a copy of the original solid?

Thanks,

Austin