Concerning the discrepancy in names over the centuries, the most probable explanation is that the original name "Lulliacum" or "Lugniacum" was later corrupted into "Nulliacum" / "Nully" by inversion of the consonants, perhaps under the influence of an old Celtic word meaning "swampy land, boggy land" (as was the land around Neuilly-sur-Seine in ancient times) which is found in the name of many French places anciently covered with water, such as Noue, Noë, Nouan, Nohant, etc. Or perhaps the consonants were simply inverted under the influence of the many settlements of France called Neuilly (a frequent place name whose etymology is completely different from the special case of Neuilly-sur-Seine).