problem setting keys of new attributes
 

I created 3 clips of animation in the trax editor that is: standing, walking and running. Now I plan to create a crouch animation but I needed to create a new attribute in his head's control box that shrinks his neck (neck_size). This attribute must be 1 in all other animations except for the crouch animation (for crouch should be 0.2).

Now the problem:
I note when I change neck_size and set keys in the crouch animation clip then it changes all neck_size values of all animations.

Remember that I'm using the trax editor to change the keys through enable/disable keys option in the context menu, but I dont know why it's not setting my desired values of neck_size for each animation I edit.

Probably I'm doing something wrong, but what?

Also, I tried to set the Source clip but the same happens, what do I do?

Help please!!!