Peculiar precedence

[parisse]

Correct, the parser has two pass: the first one (flex: lexical analyser) recognizes "tokens", i.e. x or f or ax are token of type T_SYMBOL, then the second pass (bison grammar) has rules that are applied to recognize a valid expression, e.g.
T_NUMBER T_MULT T_SYMBOL
or
T_NUMBER T_SYMBOL
etc.
At that step, the parser does not use the value of the token, only the type.
As for the documentation, I would not bet that HP has documentation on the CAS parser (to be honest, I don't think we have for Xcas, except maybe an emphasis on avoiding implicit multiplication).
It's intersting to see that the issue is raised here for the Prime, while I did not get questions like that for Xcas in France. Perhaps one reason is that you are using much more implicit multiplication?