[2023] Stats even stacking?

Stats do not stack with eachother; Each stat only applies to the weapons within it's own category.
So with Fortitude, that would be Machine Guns and Fist Weapons; But with Agility that would be Pistols and Knives.

Originally posted by Dead Inside: So what is the point of the description? why add headshot damage if there is no way to mention it in stats or does not stack, also a single headshot shot be able to take out a zombie anyways

Headshot damage is a multiplier of the basic damage value; It doesn't really need to be listed separately.
IE,
Your rifle does 100 damage, and by default does 2x damage to the head, or +100% damage. So as you raise your skills you eventually reach +300% damage, or 4x to the head, 100x4=400.

There's mods for single headshot killing zombies, if you really want it- But it gets boring quick when you can instakill any zombie with even a level 1 pipegun. Your choice if you want it that way; But that's not how this game, or many other games play by default.