The jump from Greek to Roman mythology happened in the first book of this spin-off series, "The Lost Hero". I've not got round to reading it yet but the synopsis on wikipedia describes how this series focuses on the Roman camp half blood. I would have preferred it to remain as all Greek mythology to prevent any confusion but of course, Neptune and Poseidon are one and the same.
well actualy this second seris explains why it is the greek version and not the roman counterparts survived.
essentialy they both survived but after the american civel war both sides were split aprt and hidden from each other to reduce conflict however they are needed to be reunited to prevent gia from riseing read the books and you shall understand all
The reason the roman counterparts have started to play into the books is because in the first book, the boy who it revolves around knows the gods by their roman names. Its an excellent book. I recomment reading it before you get the ' Son of Neptune' because it will scarcly make sense otherwise.