I'm assuming the writers began publishing these after the success of the novel and anime adaptations. Since I've already watched the anime series, I noticed the absences and additions of some events in the durarara manga. And since the manga series was so short, they've created this next set for everything they excluded. If you haven't seen the anime, the story is fairly entertaining, especially if you enjoyed the first set of durarara mangas. I'm enjoying so far.