Sims might or might not be correct for any particular spec/ability/proc and Blizzard only informally supports sim development.
If I were developing a Monte Carlo type sim under the conditions that an outside developer has, I would be delighted if the particular fight I was working on was off by less than 10% from the result of a number of trials by an expert player. I'm sure that many fights are simulated seemingly precisely, but only because some results will randomly be less wrong than others.
Sim DPS is really useful only as a way to determine stat weights. As a way to gauge individual performance, not so much. As a way to rank specs, not really at all.