Witcher 2 backstab damage calculation

Does anyone know how the backstab damage is calculated in the Witcher 2? I have tried testing it in the arena by having a raw character with value of the attribute "Backstab damage multiplier" being 1. The damage specified on the character screen is 9-13. So if the multiplier is 1 I suppose that the backstab damage is simply doubled, so if I hit a backstab with rolled base damage of 12 the full backstab damage would be 24. Where it gets confusing is in the log I sometimes see damage of 25 or 23 which makes no sense if the damage is doubled. Does anyone have any info on witcher 2 damage calculation?