http://lucy.allakhazam.com/spellhist...41&source=Live
Look down near the 2002 timeline, it went from Resistadj set to 0 to -200 (what we have on p99 now).
To me, this might indicate that it used to be set to a straight unresistable up until 2002-09-04, at which point it was tuned to what we have now (a high neg resist modifier, but not straight unresistable).