Sub Category Posts

I am having problems with how posts appear in my different category pages.

I have categories set up as described below;

CAT (parent)
-SUBCAT (CHILD)

I have individual category pages set up for the parent and child (i.e.
CAT = category-1.php and SUBCAT = category-2.php)

I have respective links on site to each cat. Meaning users have the ability to click on CAT and SUBCAT.

CAT and SUBCAT link to category-1.php and category-2.php respectively and display a list of posts for each category.

My goal is to have SUBCAT posts appear only under SUBCAT.

Currently when I write a post and assign it to the SUBCAT category it appears in SUBCAT fine, but it also appears in CAT.

Is there a solution to this or is this just how wordpress works?