Mana cost on boss

Hey, I don't know if I have a weird issue or if I don't understand something.

I use Tempest Flurry as a monk lvl 90+, it costs me only 27 mana on hit.

Hitting monsters, since I have mana leech and mana on kill, everything is alright.
Hitting nothing is still alright, with no monsters, my mana regen is enough and I can sustain it efficiently.

However, against bosses, it seems like my mana costs skyrocketed and my mana drain nearly instantly.

Does boss lower the mana leech / mana regen or does it increase the mana cost of skills ???

Ok I'm just stupid. I have a cast on crit which cost 95. It seems I crit a lot against bosses.