Does uber Arbiter reduce in difficulty after you die to him or does he just not have that much HP compared to the T3 pinnacles?

I just fought him for the first time, died to the fire bombs, got a respawn (already didn't expect that, I thought uber was 1 attempt only) and defeated him then. He seemed to have noticeably less HP on my second attempt, but I still got a Prism of Belief after the fight, which I think is locked to the higher difficulty. Did I just crit him harder than I thought?