Scaling mana regen Glyphs

At t3 it says that it gives +0.6/5 sec per level, which is less than the +0.65/5 sec given from the respective seal. Despite that, the total listed for the glyph is +1.2/5 sec at level 18, and +1.17/5 sec at level 18 for the seal. Something isn't adding up here.