Smooth Wash diffuser & D40 diffuser front


maybe a dumb question but i'm still curious. With the smooth wash diffuser you have to put the mirroring side towards the front, while the D40 diffusers I use for the colorsource par states that the mirroring side has to go towards the fixture.

I haven't notice a real difference with side you put towards the front except when you look towards the diffuser.

Anyone can shine a light on this?

Kind regards.