I want to calculate the volume of a model.

I believe the volume should be the same as the area since I have extruded the face with 1m but it shows a different value.

I suspect it is because it consists of loose parts. I hope the addon is able to still calculate the volume, it does it right with the area calculation.
If so, does anybody know how I can still generate a good volume calculation with the addon when an object has loose parts?

[![enter image description here][1]][1]

[![enter image description here][2]][2]


  [1]: https://i.sstatic.net/wiT91VSY.jpg
  [2]: https://i.sstatic.net/GD9ZXhQE.jpg