The idea was that, since the drive head doesn’t pass over precisely the same area on the track every time it writes to the disk, the track has to be wide enough to account for this lack of precision. Thus, a single overwrite pass could leave a portion of the track’s edge unchanged and readable with an electron microscope or something.

I looked into back in the day (but not quite as far back as that), https://alicious.com/secure-drive-data-wiping/, couple of references there.

Low density data, paired with not very accurate head tracking.