I got confused. My friend showed me a screenshot of Thall’Nosh, the Unraveler with 18m health in SR 30-31, but I could not calculate this result. I found that 2 phase boss in SR were specially reduced their health. Thall’Nosh should have only 9m health in SR 30-31.
The only possibility is that Thall’Nosh in SR did not use its SR data but its main compaign data, which had ~100% more base health.