Gamma index in 3D slicer

3D slicer version: 5.2.2
Hi every one
I have 2 RT Dose. In the 3D Slicer software, using the gamma comparison module, these dose distributions obtained from the TPS have been compared. In this process, the CT images imported into the TPS have a slice thickness of 5 mm , and the analysis indices were 3%-3mm.
considering that in these conditions, the 3mm index is less than the spatial resolution of the CT (which is 5mm), how does the 3D Slicer software perform this task?

Thanks a lot