Harvard oxford cortical and subcortical regions

i just came to know that most of the regions in harvard oxford cortical and subcortical atlas share the same voxels but the name of regions and labels are different in both, like label 7 in cortical is label 2 in subcortical atlas. why this happens? are these two are overlapping atlases? if i want to select the disjoint voxels for both cortical and subcortical atlas then what to do?

Thanks in advance for your valuable feedback.