There have been a few versions of this tool in different versions of ATK, but if you have the "GridValues" version (the latest one), then please note that you should not use Grid Values as object_id, just make sure the object is of this class (and it will be, if it's evaluated as a grid minus another grid). In fact, if you use the same object_id multiple times, you overwrite the object in the NC file each time.
So, double-check the version of the analyzer script vs. your version ATK, and don't set the object_id yourself.