Overall score comparison between 2 versions is not an indicator of strength improvement when it comes to strong engines, especially when the score is close.
When the score difference is very high then you can be sure that there's indeed a significant improvement.
In this case, not much improvement of SF 2 over 1.9. (Strategically)
Tactically, there maybe improvement of reported 20 elo.
So I'm guessing progress made in Search rather than evaluation.
Logo made by Ulysses P (Vytron)
Co-Authored with Dann Corbit: Strategic Test Suite