As far as I understand it's because in practice there is no one using pure common or pure civil law

Pure common law would be the complete absence of a separate legislature. Everything would be just layers of judicial decisions

Pure civil law systems would have judges always ruling exclusively on the case in front of them based exclusively on the text of the laws. But in practice especially at the higher level of the courts and especially in matters of constitutionality or higher laws you have a certain deference to precedence