First, sorry for my spelling. Can't seem to overcome autocorrect sneaks.
My gathering is that muun uses some behind the scenes stuff that isn't actually LN but something with submarine swaps and on chain, so the fees seem fine for tiny amounts but can be erratic.
I'll try and find some links to Stacker news articles I was digging through and put them here, and again not trying to dissuade you from muun. I used it a fair bit.
I'm still very much learning and not one to put too much credence in. Just a pleb.