Cannot change copied keywords for only one selected image

Hello,

I copy a compilation of keywords from one picture and add it to all pictures in the same library folder which is no problem. But when I add or remove a keyword in one of these images in the folder, C1 make the changes to all pictures in the folder in the same way! Usually I have not the same type of images in a folder (I don't work in sessions). Why am I not be able to define separate keywords for every single selected image?

Thank you