I think that that you are hitting the nail on the head. It is not the disk interface that is causing the difference, but the internal layout of the container file - img vs mgt.
The more general purpose img files are laid out in LBA format:
In the LBA addressing scheme, sectors are numbered as integer indexes; when mapped to CHS (cylinder-head-sector) tuples, LBA numbering starts with the first cylinder, first head, and track's first sector. Once the track is exhausted, numbering continues to the second head, while staying inside the first cylinder. Once all heads inside the first cylinder are exhausted, numbering continues from the second cylinder, etc.