Anyhow, I did mention that it is possible for items to have had a procmod that was changed prior to this discovery. This could very well be the case for Mistwalker and other items.
I do believe that items such as BSH and BoneThunder Staff received their procmod when the spell system was heavily revamped. These changes appeared to be automatically applied as opposed to applied to individual items. I will delve into that a bit later in this post.
Lets look at both those examples:
BoneThunder Staff's proc (Ward Undead) went from a level 4 cleric spell to a level 2 cleric spell where the damage went from 40ish to 12ish. Alongside that, the proc rate went from 0% to 350%.
BSH is probably the most visible of this change since it shares its proc with other weapons. Every item with the same proc as BSH (Winter's Roar) has almost the same procmod. This includes an item that procs Winter's Roar that may not even be obtainable in game.
http://lucy.allakhazam.com/spell.html?id=509
Personally, I don't know why there are slight variations of the same spell in proc rates. This could be related to the level the item procs at or which class can equip it or some other factor, I'm not sure, but
it's pretty clear in these cases that it was the spell that caused the item to receive the procmod and not the other way around.
Another thing to keep in mind is that around the time of this change, many items with classic spells (spells that had their values changed) also switched from the casted version of the spell to another version of the spell that was not castable. In other words, the Lifedraw procced by a weapon was different than the Lifedraw now casted by the necromancer. As such, those items kept their proc the same as well as their mod the same. The procmod changes appeared to be only on weapons that kept the original spell.