Strange catalog price rule behaviour

[NerdUP]

I am having some weird problem with my Catalog Price Rules.

I want to have a special discount period on all items within 1 category of 10%.
The rule was set to include all subcategories as well. All items get discounted as expected. But the problem is that some items get discounted to 19% off. 

I have no clue why it is doing this. There are no other catalog price rules or specific discounts on those products that get the 19% discount. And they are not from one specific subcategory. They are however related to different subcategory inside a different main category.

Example:

Category 1
Subcat 1
Subcat 2

Category 2
Subcat A
Subcat B 

The discount is on all products within Category 1 and its subcategorys.
They all get 10% discounts but items from Subcat A that are also located in subcat 1 or 2 get 19% discounts.

Does anyone have a solution for this? 

[Mboulliung]

You will notice that 19% discount consists of having 2 cumulative 10% discount :
100*0.9*0.9 = 0.81 = 19% discount.
Consequently, i believe, depending on how you setup your rule, that you should select ONLY categories or ONLY subcategories, but not both, as this makes the discount happens 2 times.