Now that you know a little more than just the basics, I'm sure you can see why one company's 550W might out perform another's when put head to head. Or even more a valid point, why upgrading a 300W to a 600W may not yield any better performance.
With so many variables like temperature, the de-rating curve and how amperages are distributed across different rails, it's easy to see how two seemingly identical power supplies can actually be quite different.
Hopefully I've turned you on to a few more features to look for in a power supply, like looking at a power supply with power factor correction and better efficiency than another. Or perhaps helped clear things up about resistance and PSU cooling. I realize it's a lot of information to digest, but considering I haven't told you nearly everything, like independent voltage regulation and crossloading, I think it's a good start.