Nine tails is suppose to scale to the playerbase just like GNW i think.

On my server Monday night nine tails it's health dropped 120 million since 2 of the Top 3 players for it were absent on Sunday however after joining in on Monday the health not only went back to 398 million but as of tonight (Wednesday) increased to 418 million now.

So it's been different for 3 days straight.

If you wait a day or more the game may have a statisic to scale it's health to.