It all have to do with the "low-level" output of the RCA connector. For a number of manufacturer, the output voltage is approx. 1V, for others, 0.7V, for some, as low as 0.5V. I have seen 1.5V RCA output, although rare...

It would be great if all of these components (CD, DVD, ...) have variable outputs, so the level can be adjusted to match the expectation of the receiver, but I guess we can dream...