It is for all practical purposesthe same grape. Traditional naming convention lists the grape as Petite Sirah in the United States, however that seems to slip depending on who is printing the label. In other parts of the world, it's known as Durif.
There is most assuredly a difference between Petite Sirah and Syrah grape varietals. Petite Sirah was made through crossing Syrah with another grape type.
Why Mosti Mondiale chose to spell the name with a 'y', I do not know.