"Brutal Strength" punishes mana more than written

TL,DR:
Brutal Strength does double penalty on warriors - it reduces your maximum mana -10% AND rises skill cost for about +15%

my lvl 37 shockwave totem tooltip says it costs 937 mana, in fact it cost about 1080 mana (without any support gems, just skill itself)
it always about 15% higher, so i did some passive tree rollbacks and found that without Brutal Strength manacost seems to be correct