Flameblast mana cost calculation

I noticed that increasing my mana regen isn't doing what I'd hoped with my flameblast infernalist.

- Level 25 Flameblast skill
- tooltip states 122 mana/sec to channel
- Cast time per stage down from 0.25s to 0.11s from cast speed
- 1400 mana pool
- 267/sec mana regen while moving, 224 while stationary


when I channel the skill it burns through my mana in 9 sec stationary, and 12 sec while moving, so regen isn't paused while channeling.

I thought that channeling cost doesn't multiply with cast speed, but even if it does the +120% cast speed that results in 0.11s cast time would cost 266 mana/sec right and I'd be able to sustain chanelling?


so it seems the actual cost of channeling this is 380 mana/sec? why so different from the tooltip cost?