I thought that ss would add power on physical skills. I mean I checked it and when I use a physical skill, like power strike for instance, no SSD is consumed. This makes my physical skill attack to make less damage than just hitting the mob with SSD.
From l2wiki
There are Soulshots and Spiritshots, and there are also shots for pets/servitors. For Phys. skills and attack you need Soulshots. For M. skills you need Spiritshots or Blessed Spiritshots. Similarly, for Phys. skills and attacks of servitors/pets Soulshots are required; for M. skills of servitors/pets Beast Spiritshots/Blessed Beast Spiritshots are required. Shots are divided by Grades; the grade of your weapon and the grade of shots must match.
I think this was re-worked in later Chronicles like you mentioned, but in C3 Soulshots are only used on auto attacks and Spiritshots are used on magic skills.