Confused about stats

Help me with something: I’m confused about how stats work on OT1.

I’ll give an example: I’m equipping Primrose to be a secondary scholar. Her base E.atk is 227. If I give her the Soul Knife (E.atk +188), it rises to 415. I also have the option to give her the Sapphire Rod (E.atk +158). But then it doesn’t appear to stack. It shows her base E.atk rising to 385, instead of adding 158 to the 415 due to be equipped with the knife. How comes?