216 watts @ 12 volts(216/12=18amps combined), while enough(but still lower then the 300 watt I have with 22amps @ 12 volts) is still rather low for a "390 watt" power supply.
The voltage switch is more of an indicator of and older design(older designs are also less efficient).
This explains why it does not put a larger portion of its total power into the 12 volt rail(s). It in fact has little to no reason for rails since it only has 18 amps and that falls within the specs for a single rail power supply.
The multi rail idea is almost always just a current limiter(to separate a larger 12 volt rail into smaller rails. Think of it the same way your electrical panel works) to prevent a user from trying to run the full system off 1 plug with tons of adapters and splitters.
Newer power supplies use one very large 12 volt rail then split or do not split it with current limiters(it is always a debate of single vs multi rail).
Now the important part is that they also provide the 5 and 3.3 volt rails from a DC-DC converter board that is power off the 12 volt rail. This is why you can see a 850 watt power supply with 840 watts @ 12 volts yet still another 130 watts on the 3.3/5. So to some extent a system using lots of 3.3/5 volts will slightly decrease to 12 volt current. 3.3 and 5 volt rails are not heavily used on modern systems. ALL USB powered devices as well as laptop drives and parts of hard drives do use the 5 volt rail, but not too much of it.
These large 12 volt rails + 3.3/5 off that rail means that if you do need to use extra power on the 3.3/5 volt rails, you can and if you do not, you can use more 12 volt power.
In this example, every watt you pull from the lower rails will lower the 288 watt limit for the 12 volt rails. As you can see ALL rails together must NOT pass 300 watts on this power supply. You also note despite seeing 2 x 17 amp rails, the load from both must never pass 24 amps. Many users make a mistake and this that they have 34 amps. This is not true(34 x 12 = 408 watts)
http://imageshack.us/a/img560/4529/multirail.jpg