Determining volume of object in Blender

I am trying to create this trilobe cylinder in Blender. Using the 3D-Print Add on, I just wanted to make sure that the volume of this object is actually right and that everything is okay. However, it shows that the volume is 18728670.7467 cubic cm while mathematically it turns out to be 17762523.04 cubic cm. Why is that? Do I need to worry? I want this object to act as solid as compared to hollow when they fall from a height. Am I doing it wrong?

Also, the cylinders are each 2 m in diameter and 2 meters high. The centers of the cylinders form an equilateral triangle.

PS: Viewport Display set to Wire