Toxic Rain Area of effect

Hi GGG
My toxic rain gem has 20-20 quality-level and I should have a 10% of increased area effect.
This rate is not appearing on the stats of the caracter where I just see the 20% from bow mastery node and 9% from an item (my helmet).
Could you help me to understand why I do not have the 39% area effect and just 29%?
Is it a bug?
Thanks in advance for your help
Alex