Difference between 4x4 and 100x100mm filters

So I just got a Smallrig mattebox, however my 'regular' 100x100mm Nisi IRND filters reflect immensely in it. Now I noticed it says on the Smallrig website that it does not accept 100mm filters. However I'm trying to wrap my head around why not ... These filters are a tiny fraction smaller and seem to fit snugly in the box. What is not working?

It expects filters with a thickness of 4mm and not the 2mm of your filter.
3 to 4 strips of gaffer tape on top and bottom of your filter adds the needed thickness and height.