I noticed that Chaos;Head’s divergence ratio is different than the one of Steins;Gate and Robotics;Notes. What was the point of that series to begin with since nothing basically happened?
Because the point of Science Adventure series is not really about divergence ratio or the same worldline, but just due to the official genre/theme.
Back in 2008, when Chaos;Head was first released, the genre was officially given as "Delusional Science Adventure" (妄想科学ADV). At that time, there was no plan of making this into a series.
It was not until 2009 when Steins;Gate was released with the genre as "Supposition Science Adventure" (想定科学ADV), making it a series and also the 2nd entry.
This is then followed by:
- Robotics;Note: Expansive Science Adventure (拡張科学ADV)
- Chaos;Child: Delusional Science Adventure (the same as Chaos;Head)
- Occultic;Nine: Paranormal Science Adventure (超常科学ADV)
- Anonymous;Code: Supposition Science Adventure (the same as Steins;Gate)
The series emphasizes real scientific elements heavily while still allowing them to mix with fictional elements. This includes, but not strictly limited to, time travel and divergence ratio.
Main reference: Japanese Wikipedia on Science Adventure Series