That routine does not signify that editing has ended. Instead, it's called to find out if it should end (hence the name of the method). It can be called by the framework any number of times, and you shouldn't be relying on it for this purpose.
Instead override the NSOutlineView's textDidEndEditing: method. Be sure to call super.
So you'd subclass the NSOutlineView and in your subclass:
- (void)textDidEndEditing:(NSNotification *)aNotification
{
// do your stuff
[super textDidEndEditing:aNotification];
}