Designing a good PSU is quite an art, especially when balancing power output, stability, efficiency, cost and durability.
Often cheap power supplies will use two or more cheap 12V circuits, which are rated for a relatively low power draw. Better quality PSU's might use multiple higher quality 12V circuits, but others might have a single high quality/power/efficiency 12V power circuit.
Even high quality single rail PSU's may break out that one rail into multiple rails with individual overload protection on each supplied rail.
be quiet! PSU is probably of the last type, it has a single 41A circuit, providing power to 4 rails, where either a total draw of more than 41A or a draw of more than 18A on any one rail will trip the over-current protection.
Power LC is either an old or cheap PSU which probably has two different cheaper 12V circuits. Don't expect either rail to perform anywhere near the limits suggested.
Personally, I prefer pure single rail PSU's (where you might see 12V 41A as the only spec). You don't need to worry about balancing power over each rail and you don't risk overloading one rail when you add a new device because you've plugged it into a power connector that uses the same 12v rail as your graphics card.
The only problem with a single rail PSU is that since they can potentially supply more current than the cables are rated for, if you have a short circuit you could end up burning out the cables before the overload protection kicks in. This is still very unlikely though, so much so that respected PSU makes are happy to sell single-rail PSU in the consumer market.
If you are unsure about whether your PSU is suitable for your system or not, I've found the eXtreme Power Supply Calculator Lite v2.5 to be an interesting first port of call. Just don't forget to add in capacitor aging factor (see note 4) if you intend to run your computer 24/7 or want it to remain reliable for more than a year.