Most of the Cube space has value 0, because it is not brain tissue or it is not labelled in the Atlas.
Make a test by opening the atlas, selecting the a voxel, get it's value, label and MRI coordinates, and try to retrieve the same information with the code. For example in this image, the voxel is (120, 116, 151), value 2, label Right Precentral gyrus.
That's correct