Transmutation Mana Reduction

I have no idea what else would be affecting my mp cost. I made sure meditation was off and I did the casting during the day to minimize the effects of mp recovery when noting starting and ending mp. I was wearing nothing with set effects and all enchants were related to magic damage or crit. I was using a Casting Speed +40% -6% Damage Phoenix Fire Wand with no gem upgrades.

I have heard rumors that every 100 int contributes to a 1% mp reduction which would actually explain why a single charge of fireball, which would normally cost 20 mp with rank 1 Fire Mastery, is costing around 15 mp instead since I have 970 int with that wand on. Perhaps I should attempt to verify or debunk that rumor with the help of potion poisoning.

EDIT: Wait no it wouldn't... even if I round up, 10% of 20 is 2 mp and I should be using 18 mp per charge.