NPCs have not had the purity ability up to now, but these guys have the code - having said that, it may mean taking them from the main item generating routines. Might be a big task.
In terms of the 97% failing 40 times... whilst logically it has a cat in hells chance of happening, it can still happen. I wouldn't buy a lottery ticket if it happened to me, but hey

Stats being as they are, failing 40 times for a small group of people, even one person, doesn't mean elsewhere in the game, the stats are giving 200+ successes to other players - who of course won't 'complain'.
The only way I can think to be sure the engine is actually working for %ages is to log them game-wide and over a period of time.
(as a side note, things like adding stats with benes, orbs, etc, previously depended also on whether the item already had a stat added to it also, and some variable regarding the player (like bodyluck). so as opposed to being a flat 97%, it'd be 97%+luck+existing item status. Can't say what it is now cos I've not filtered the code the guys are using)