Oriented bounding box estimates are quite off

### 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](http://www.open3d.org/docs/latest/getting_started.html#development-version-pip).
- [X] I have checked the [release documentation](http://www.open3d.org/docs/release/) and the [latest documentation](http://www.open3d.org/docs/latest/) (for `master` branch).

### My Question

Hi, I have a partial point cloud, and a point cloud of better quality. However, when I run the oriented bounding box by calling pcd.get_oriented_bounding_box(robust=True), and visualize the results for both point clouds, the estimation for the better point cloud are even worse than the more partial-viewed cloud. I'm curious about how this is happening. Any suggestions and ideas are greatly appreciated!