Where are you reading the voltage? The XM13A has TWO outputs. One switched and one always ON.
If you are measuring the voltage between the switched supply wire and the constant supply wire.
When you turn it On. The voltage between the switched and unswitched wires will be zero volts.
When you turn it Off you will read the 12 volts. Making Off look like On and On look like Off.
So between the common ground wire {maybe a shield around the two voltage wires} one should always be 12 volts and the other wire to common should switch On and Off 12 volts.
If there is no load on the switched 12 volt wire. Your meter may not go to zero volts when Off. The switching transistors leagage. Maybe enough to have your high impedance meter reading some voltage but you should see a difference On and Off.
The supply is not regulated. So the output maybe higher with smaller loads and lower with heavier loads. My XM14A from a Ninja Base. Is about 25.2 volts unloaded and drops to the 12 volts when loaded to its rated 200 mA.
http://kbase.x10.com/wiki/XM13A