Earlier versions of HD Setup initialized a volume to its maximum formatted
capacity. Drives of similar advertised capacity have slightly varying
total formatted size.
The release of the Apple Tape Backup unit lets you do a mirror-image backup
of your drive. But in order to restore a mirror image backup, the volume
that is being restored to must be of like or greater capacity than the
volume from which the data came.
If, for instance, you had a failing hard disk that could be formatted to
78MB, did a mirror-image backup, had the drive replaced with one that could
be formatted to only 76MB, you wouldn't be able to restore your work.
Apple modified HD Setup so that it formats each series of drive we sell to
a like-total formatted size for that series, regardless of vendor. In the
case of the nominally 80MB drive, 76MB is the lowest common denominator of
all vendors that we use.
You may be able to reclaim some of the unused 4MB by using a third-party
initialization application, but this is not guaranteed nor supported by
Apple.