It means that If you play enough time, chances are you would miss 100 times with unit that has 5% chance to miss.....
Wat. No idea what you are saying here.
The only RNG element involved with Nades, is this one:
Which states that only on models that have <50% you have a 10% chance for a kill crit.
Throwing R guard nade at MG42, 5-6 separate occasions did just about nothing to mg42 in the building.
If 5-6 Nades did "just about nothing" then you must have thrown them somewhere that they did not hit the MG42 models. Has nothing to do with RNG, only with placement.