If the base mana cost of Mana Storm is 20, and 2 points in Explosive Flurry gives -40% mana cost, then that should reduce the cost by 8 (20 x 0.4), not 5, giving a cost of 12.
That is just a tooltip bug, the Tooltip Costs of Mana Storm always say 15, regardless of what nodes you took. It does not change at all in the tooltip.
Also while being irrelevant for your point, i looks like you specced into the follow up nodes, which modify the mana cost even further.
I know that the base mana costs are 20, which works as intended and if you specced all 5 follow up nodes in “Explosive Flurry” + “Essence Of Control”, Mana Storm does not cost any mana at all.
Ok, it’s a tooltip bug then, that’s fine, still needs to be fixed.
New in patch 0.7.10, yes it’s a tooltip bug.