I think I do know why it is. I'll try to explain :
Think that u decrease someone's pierce resist by 1000, his armour is still 1200 as well. And so, his pierce resists sits at x-1000. Now u shatter him taking down 600 of his armour, but it doesnt add to the pierce resist that was already taken (Im sorry I dont have the proper words to explain, never used them), it simply goes into the x-1000. But, if pierce lure took down just 100, and its x-100, then the 600 shatter would not stack but replace it, making it x-600.
I doubt I could explain, but its as if the larger debuff is the one that decides.
Armour is nothing but piercing slashing crushing resistances. 1 armour is 1 slash pierce crush res, so if a mage casts pierce lure and lowers boss pierce res by 1000, a 800 shatter won't lower his pierce res by a further 800, but will lower the non lured resistances like crush and slash. The issue is no one uses axes,swords hammers to dps a boss, so is entirely up to you depending on which group is there. If you have good amount of slash and crush dmg at the fight then shatter will help.
These statements above are completely wrong.
Shatter does stack with pierce lure. It lowers pierce resistance even more after pierce lure is already in effect.