Chance to poison not stacking?

I have selected two passives that give chance to "poison on hit" and two passives that give chance to "poison on hit with attacks" (with bows) - as shown in the following skill tree:

https://poeplanner.com/AAkAACkACQIAABGbjc6oAbzveu4OVvp31xR1GNs5JSxkqqT2oxUgtQixMCU_AAAAAAA=

I kinda assumed that when I perform an "attack" skill with my bow that would mean I will have 40% chance to poison in total (10% from each node, 2 of each nodes).

But somehow I only have 20% chance to poison. (at least the character screen says so - its kinda hard to test since the 30% chance to bleed sometimes "hides" the poison procs)

Does anyone know how exactly it works then? Because it seems completely counter-intuitive that these two things dont actually stack.

Same issue here. My passive skill tree thus far looks something like this:

https://poeplanner.com/AAkAAIkACRICAEGbjQgu7YMZjgBe0359df66u-PEopcGGYrDOiLq6NYFtQZwk5lOn3p_73rnCgqbeOv8xYAiZKY_bPajFSC1CLEwJT9xoY9PLYMZtL6nZU29NnXL6mK3to1-QzHDM1oav9VTu90NW69OKiPTbyeaao9g4J8FZtZYFHUY2zklLGSqpOl2AAAAAAA=

I've taken all the "Chance to Poison" nodes (except for the dagger ones in the Shadow's Dagger circle). My Chance to Poison on my character sheet should intuitively be 80% with 'Attack' skills. However, as you can see in the below screenshot, it is only 60%.

http://steamcommunity.com/sharedfiles/filedetails/?id=1105865856

It can also be noted, that the missing 20% shows up as a separate modifier.
The character sheet does not indicate that 20% from attacks is even applied at all.

Same thing here. Chance is not stacking. Well sometimes my barrage will show one of the 10% nodes to poison, sometimes it doesn't. Weird.

Anyone know if this is intended???