LayerMask detecting layer it does not contain?

Hi there,
I have a public LayerMask, in the inspector. I set it to hold two layers. My issue is that, when using Raycast, the ray hits a third layer. I do not want this.

Code (csharp):

1.  
2.  
3.                 Ray ray = camThis.ScreenPointToRay(Input.mousePosition);
4.                 RaycastHit hit;
5.  
6.                 if(Physics.Raycast(ray, out hit, lm_plate)){
7.  
8. }
9.  

* lm_plate contains layers x and y, but at hit occurs with layer z. Why? How? Thanks