Halliday & Matthiessen (2004: 570):
But the most important distinction … is that in ellipsis the typical meaning is not one of co-reference. There is always some significant difference between the second instance and the first (between presuming item and presumed). … Thus reference signals ‘the same member’ (unless marked as different by the use of comparison); ellipsis signals ‘another member of the same class’ (unless marked as identical by same etc).