A Floor Wall operation is being used to cut a through-pocket feature. The floor is picked as the planar face at the bottom of the part where the pocket breaks through. The tool path has to cut below this floor by a small amount to ensure full cleanup.
Floor Wall milling is designed to cut to a floor face, like the solid bottom of a pocket. However, there are ways to make it cut beyond the bottom of a through-part feature.
1) The documentation for the floor wall operation contains an explanation for the Z Depth Offset parameter. This field adds an "implied offset below the bottom edge of the selected wall..." It's not an offset of the floor; it's an offset of the bottom edge of the wall. In the floor wall operation, select only wall geometry. Let the software figure out where the floor is supposed to be based on the bottom edges of these walls. Under this condition, the Z Depth Offset parameter will be applied to push the passes that distance below the bottom face of the part.
2) In case the part does not have a nice clean bottom face where the walls all end in the same plane, a separate planar surface can be created at some distance below the part. This can be made using normal NX modeling tools or Synchronous Modeling right in manufacturing. This new face is added to the Part class of geometry in the workpiece parent, and then selected as the 'floor' geometry. This will cut the walls down below the actual part to the new separate floor face.