Can you check that you are using the latest release of dcm2niix
  https://github.com/rordenlab/dcm2niix/releases
If that does not work, I suggest you press the "Issue" button on dcm2niix's github page and create a new issue. Also, you may want to make sure that the software you are using to measure these values uses the NIfTI scl_slope and scl_inter values. The raw data in the NIfTI image is the same as in the DICOM image, and the scl_slope and scl_inter come from the DICOM header (rescale intercept (0028,1052), rescale slope (0028,1053)).