"Knockback" support gem not giving "40% more knockback distance"

I have 2 jewels for "increased knockback distance", one 10% and another 15%, total 25%.
In my "Leap Slam" skill gem i see the same value - 25%.
When i add "Knockback" support gem to my "Leap Slam" knockback distance modifier is not changing and stays at same value of 25%.

Screenshot without support gem:

Screenshot with support gem:

P.S. When i add third jewel with 10% knockback distance, this value changing to 35%.