The ratings you mentioned are the 120VAC ratings.  The contacts have different make/break ratings.  In other words when the output is energized and the contacts close or 'make,' the contact can handle an inrush of 15A.  When you go to turn off the output and the contact has to open or 'break,'  the contact should have no more than 1.5A of current accross it.  Any time you break a circuit, you get arcing.  Different contacts can handle greater or lesser amounts depending on their design.