What is the difference between a SCART-RCA IN adaptor and SCART-RCA OUT?

I want to get the audio out of the SCART output on my freeview box to plug into my amp via phono/RCA. I have a spare RCA to SCART adaptor from my PS2, which I thought should work but I've looked it up and there seems to be 2 types of adaptor depending on direction? I would have thought the same adaptor should work both ways - is that not the case and (just out of curiousity) if not, why not?