Because the top layer of glass is a size larger to help avoid vignetting.
The front threads of Variable ND filters are a size bigger to avoid any vignetting. This also means your lens’ original filter cap may not fit on your variable ND. All our lens filters are double threaded so if you want to keep your Variable ND filter attached and use a lens cap, you just need a lens cap that is one size larger. See the handy Variable ND filter cap resizing chart below. You can use one of our filter kit caps, which can be used as a lens cap. Otherwise, you can remove the filter and use your original lens cap.