Materia VI stronger than Materia VII

[Scox]

Materia VI gives +40 yet Materia VII gives +20 and Materia VIII gives +60, this doesn't make sense. Materia VII should be stronger than Materia VI, or am I losing my mind here? I've melded Materia VI to my 440 gear and it goes in just fine. I thought it was a tool tip error, but it's not.