It's the literary equivalent of the auteur director in that it is what is unique about the author/director that makes someone buy just because its them. E.g., I love to read anything by the late great Maeve Binchy. She didn't do series, but she wrote a style of fiction I enjoy and her skills in writing dialogue are wonderful and inspired me. So yes it is more than just style. It is what would make the book recognisable as yours even if in a completely different genre.