Making sense of power supplies

[enzo matrix]

I need someone with experience in computer power supplies to help me make sense of the calculations at play:
-Why is the total wattage of the PSU not equal to the total output power of each of the rails?
-why are two +12V rails at 25A each equal to one +12V rails at 42A?
-If the manufacturer gives me the total wattage of the +12V rails, can I safely use this figure to calculate how much power I will be able to use for the attached components?

Example, scroll down to features:
http://ncix.com/products/index.php?sku=33041

 

[delluser1]

1 and 2 ) Because the rail ratings show only what they are capable of seperately and are not additive.
3) Yes

 

[enzo matrix]

For my question one, I am referring to the second from the bottom vs the bottom wattage in the chart on my link. Just to double check, this is still the reason why?
Thanks delluser1. Anyone with more detailed information?

 

[delluser1]

http://www.jonnyguru.com/forums/showthread.php?t=1036

 

[enzo matrix]

Best answer selected by enzo matrix.