The common name of grass-of-Parnassus comes from the European species. It was common on the slopes of Mount Parnassus in Greece. Mt. Parnassus is limestone, and these plants live in limestone rich habitats. It is not remotely related to grass. But to paraphrase one author, (http://www.plantlife.org.uk/wild_plants/plant_species/grass-of-parnassus/words) it became an honorary grass because cattle liked it so much. The technical classification is currently more murky. My preferred source puts Parnassia in its own family, Parnassiaceae. Another has it still in its traditional saxifrage family. Yet another would place it in the bittersweet family. Modern methods can be very precise about plant relationships. But while that process is ongoing, confusion abounds. I'm sure they will ultimately pick one family, and sometime after that, the references will catch up.