What if it's already 100? With +300%, now it's 400. Pretty sure there's a limit anyway, despite what the numbers say. I know that's true for armor value. Even with the number in the trillions, you still only block up to 85% of the total incoming damage.
It's not as fun as Oblivion or Morrowind where you can do shit like make enchanted armor that, in total, Reflects 125% of all incoming damage, meaning you are immortal and everything attacking you takes more damage than they output. 🤣
Good ole alchemy enchanting loop. Make an outfit with the fortify alchemy enchantment, then make a fortify enchanting potion, then make another set of fortify alchemy, rinse repeat.
Tbh, those are rookie numbers. You can get them WAY higher if you cheese the enchantment loop exploit more aggressively. And I mean like millions of percent.
My guess is a mod. If you're not using mods (which specific ones being a matter of taste and playstyle), you're not doing Bethesda games right. That's not just me saying it, it's the law.