How convenient that the reward for the first boss she fought happened to be perfect for her build.
As others mentioned, that probably has to do with the fact that both girls solo'd the bosses they found the first time they fought them. I remember it mentioning that for Mable to some extent. Some games do have targeted drops to varying degrees as well, like if you're playing a class you kind of want gear for that, not other classes, and sometimes the game will help you with that by dropping stuff suitable for you in particular. Granted, getting anything really good is still a matter of luck usually so whether it's actually an upgrade or not is still up to chance.
We can also see from Sally that luck may have played a role on this bonus loot for first time boss soloing, since Mable's stuff was smaller in quantity and all but vastly superior. That stuff was like the devs decided to make some of the best equipment in the game available at low level and function like heirloom equipment, so it grows with your character. That was either crazy luck or the nature of the game detecting their skill and stat distributions, and rewarding Mable a bit more than usual because she really took a chance that could have horribly backfired on her to build her character this way.
It wouldn't be unusual if her build had resulted in her initially being resistant but not immune to damage, but basically incapable of causing any damage to anything herself because she can't cause enough damage to negate anything else's defenses. Honestly, that rabbit killing itself attacking her was nothing short of plot armor, unless they can run out of the ability to attack normally and have to "struggle" like in Pokemon or something when you use all your move PP. The struggle attacks cause recoil damage to sort of prevent people from just spamming it and go recover one way or another before fighting more.
