I can kind if answer that. So I use a power supply that I designed, and I'm pretty sure it operates the same way theirs does.
My power supply takes a 12V 4A input. It has 10 outputs, and each one has its own isolated regulator. Each regulator is rated for 1.5A, so in theory any output can drive a 9V 1.5A load, but they are all sharing the 12V 4A input, so the sum of their current still has to be less than 4A. Isolated outputs, but with a shared input. So even though each one is capable of 1.5A I can't label all of them as 1.5A outputs, because the input power supply can't handle 15A. But to certify something for consumer use they're required to label the current of each output, even if the output limit is actually "it depends."