Rain of Arrows +4/+9 Weapon Damage doesn't scale

[panvxv]

The extra damage while using Rain of Arrows doesn't scale at all with +% Weapon Damage.
I have tested it with Sharpshooter and Basic Magazine.
Basic Magazine (+15% and +6%) didn't increase the damage of the +9 Weapon damage.
And Sharpshooter (+70% + 14%) didn't increase the damage as well.

As you can see in the picture below the difference is exactly 9. If items were effecting this extra damage it would be at least 15.