No that's what I'm asking. It's 15% for any item that drops (or coined) to be from the shared loot table (of your spec etc), so no matter what boss you coin if you're lucky to get a "loot" then regardless of boss that has a 15% chance to be from the shared loot table. I can't find any information on it on official forums (blue posts) but in my mind this is how the shared loot works.
Boss drops 6 items (3 if tier boss) each of those 6/3 loot "slots" has a 15% chance to be from the shared loot table.
Don't know what the coin loot rate is but if you get loot that item should also have 15% chance to be from the shared loot table.
I just don't see why (and how) a boss with 4 items for ret spec to have less of a chance to reward you with something of the shared loot table (again 15% on any loot piece) the only thing you're doing is if it's loot you're guaranteeing to either get that 1 shitty ring (magaera) or one of the 3-4 items off the shared loot table.
I can't imagine it works like this.
Coin Magaera - you get loot, now since there's only the ring for rets we get the mag rings average drop rate say 15-20%?
Magaera ring 15%-20% drop chance.
Shared loot table loot 80%-85% drop chance.
oh and btw, used all my coins this tier so far. gotten prim cloak (which I'm using), ji'kun chest (vendored) and that's it. neither were HC TF.