Regarding Unique Boss Drop Protection

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In the campaign, you only get the guaranteed Rare item the first time you kill the boss. In maps you always get it.

I'm not sure I understand this.

So suppose I hit a wall in say Act 3 because I've not seen any upgrades since Act 1. The logical thing to do is to go back to farm Act 2, since in this example my gear is just not up to the task for Act 3.

But I've obviously killed all the bosses in Act 2 already. So now when farming the bosses in Act 2, in hopes of improving my gear, the game is designed so that I can get worse loot than the loot that already failed to be upgrades.

Why would a "protection" against bad luck be designed so that RNG can give me even worse loot than before?