That means that all the files have Big Beat in common. Any tag that is not in all the files (or not arranged the same in all files) is not shown there. However, if you open the tag inspector you will see all the tags from all the files. Any changes you make in the tag inspector (removing a tag, editing a tag, etc) will be applied to all the files, regardless of whether it had that tag to begin with.
For instance, if some tags said Electro and you edited it to say Electronic, all files would now have the Electronic tag, regardless of whether they had the Electro tag originally.