Great vault Question

I was always under the impression that your vault would fill with loot from only the bosses you have killed up until that point.

Eg if you have only killed the first four bosses in heroic you only get loot from those four bosses on the loot table.

Has this changed recently or was I just wrong the whole time?

We had a guild member get the trinket from the turtle boss on myth track in vault this morning and he has never killed it on heroic since patch launch.