[Scrubosaur]

Ya it does look like it is supposed to be a flat reduction in mana usage :

Effect on Mana Consumption

Specialization skills passively reduce the amount of mana that is consumed when casting spells from that school, according to the following formula:

1 + (Skill_Value / 20) = Percentage_Reduction

...where "Skill_Value" is the skill point number that the character has in the associated specialization skill, and "Percentage_Reduction" is the perentage of mana that will be preserved.

Thus, for a given spell school, a character gains 1% mana cost reduction just for having the Specialization skill at all, and 1% more for every 20 points of the skill.