Diff with the previous one is a different command than viewing what has been changed in your working copy.
The previous one is defined as the version prior to the version that you have in your working copy:
A revision argument can be one of:
NUMBER revision number
'{' DATE '}' revision at start of the date
'HEAD' latest in repository
'BASE' base rev of item working copy
'COMMITTED' last commit at or before BASE
'PREV' revision just before COMMITTED
PREVIOUS, COMMITTED