How did the Old French word romanz change from meaning vernacular language to meaning a narrative work?

The Old French word romanz originated from the popular Latin term Romanice, which referred to writing in the vernacular—the "speech of the people"—as opposed to literary Latin.1

The transition from describing a language to identifying a specific narrative work occurred as the term began to be applied to the texts themselves. Initially, it was used to denote a French-language version of a work, such as Wace of Jersey’s Li Romanz de Brut (an adaptation of Geoffrey of Monmouth’s Historia regum Britanniae). Over time, it became ambiguous whether li romanz referred to the "French version" or the "story" itself.1

The word eventually specialized to mean the narrative composition, particularly those involving chivalric themes, and moved away from its linguistic roots. As these compositions evolved and diversified, the term acquired a broader spectrum of meanings, eventually leading to the modern sense of a "novel" in French or a "romance" in English.1