Why mesh volume measurement inaccurate?

### Checklist

- [X] I have searched for [similar issues](https://github.com/isl-org/Open3D/issues).
- [X] For Python issues, I have tested with the [latest development wheel](https://www.open3d.org/docs/latest/getting_started.html#development-version-pip).
- [X] I have checked the [release documentation](https://www.open3d.org/docs/release/) and the [latest documentation](https://www.open3d.org/docs/latest/) (for `main` branch).

### My Question

I have a mesh of known size box.
Mesh is created using Meshroom's photogrammetry pipeline.
scale is 1 unit = 100mm

Using Meshlab's measure tool, I measured the height, width and breadth.
Below is the image showing the measurements (1 unit = 100mm)
**The volume is 2.3 unit cube**

Also I measure the volume using Open3d's mesh volume attribute. Here I used bounding box volume as the mesh is not watertight.
**The volume is 8.9**

Why there is so much difference in volume calculates using both methods?