I spoken about dice rolls before and the possible wipe out effect so your throwing my words back at me. I could choose an aggregated performance factor of 50% or 25%, designed to wipe out your more average dice roll numbers - thats what happens with extremes. Thats what can happen with Trolls or highest training numbers - spectacular in their effect but they're not the norm Most of the time things muddle along in the mid-ground where even then small gains can be lost but also where the aggregation of small gains have the biggest impact.
My simple point which I've also said before is that I think of this game as footballs version of pnp dnd - a myriad of hits n misses, bonuses and penalties, boosts n nerfs, adjustments, defence and atach blah-de-blah, (with the DMs Commentary providing hints and immersion). Most of the time these small things combine to affect the outcomes - but a Natural 20 (or 1) is always a possibility.