I am well aware of how a PSU works and what makes a good one thanks
The fact is, regardless of whether its 'rated' as having good 12V rails, it doesnt neccessarily mean it can deliver that performance.
The way to determine TRUE amperage is to take the continous delivered wattage NOT the rated wattage (which is the peak maximum).
Divide is by 12.
This equals the TRUE 12V amperage being delivered.
I will give you an example as you got a bit lost.
Enermax Liberty 500W. - Good example as i have it on the shelf next to me.
Rated amperage - 500W
TRUE continuous amperage - 488W (12V Rail)
So...if it really was 500W then it would be:
500W/12V = 41.66W
41.66V / 2 (12V Rails) = 20.83A per rail
Now/........the TRUE amperage is:
488W/12V = 40.66
40.66V / 2 (12V Rails) = 20.33A
Those figures are pretty damn close to each other but 'less interested' PSU manufacturers massage these numbers. THe ONLY number that cannot be hidden is the TRUE amperage figure and this is where many fall down.