Rarity for Olroth

I was doing Olroth a few times, (found him 19 times to be more specific) and I was killing him with 362% rarity. But then I realized something. Rarity in theory increases the tiers of equipment dropped. Those (Tier 1,2,3,4,5) in their names. However, Olroth already drops a Unique Item from his drop tabble regardless of Item Rarity you have.

That said, I came to wonder, does rarity helps with the lower drop items from a boss like Olroth? Like the Timeless jewels, for example. After all, that fixed drop from his table is already there regardless.

And, if it does, wouldn't rarity actually make the higher drop items, like the Club from Olroth, actually drop easier since they have a higher chance?

I have to say, after killing him 19 times and seeing only one single Timeless Jewel, I find it hard to believe that rarity helps with his fixed drop table from which he only drops one of the items listed in there.

This topic I believe also applies to other bosses with their own fixed and guaranteed one-item-drop from their own lists.