Why is cake a referent of good?

Why do we have words that mean things at all?

Why does it appear to make sense to wonder if we are valuing the right things

For a start, the fact that some things seem to make sense is not a oracular window unto philosophical truth. Anything that we are unsure about will seem as if it could go either way, even if one of the options is in fact logically necessary or empirically true. That's the point of being unsure (example: the Riemann conjecture).

At the object level, no-one knows in full detail exactly what they mean by "good", or the detailed contents of their own values. So trying to test "my values are good" by direct comparison, so to speak, is a highly nontrivial (read: impossible) exercise. Figuring out based on things like "wanting to do the right thing" that "good" and "human values" refer to the same thing while not being synonymous is another nontrivial exercise.

I don't see the S/R difference is relevant to relativism. If the referents of "good" vary with the mental contents of the person saying "good", that is relativism/subjectivism. (That the values referenced are ultimately physical does not affect that: relativism is an epistemological claim, not a metaphysical one).

To me, the fact that you don't understand is evidence the difference matters. Unless you're saying that "relativism" is just the statement that people on different planets speak different languages, in which case, "no shit" as the French say.