This is because you have not removed the tag from the contacts that it is applied to.
The tag is no longer a global tag as you have removed it from the global tags box - but instead it has become an ad-hoc tag.
Because this ad-hoc tag is still applied to contacts that is why you are still seeing this tag when you click on the tag label icon to search your tag groups.
The best way to deal with this is to firstly make the ad-hoc tag once again a global tag and then remove this tag from the contacts that it has been applied to - that way the tag is no longer in use.
Once this has been done you can then remove the tag from the global tags box - this way you will have removed the tag completely and you will no longer see the tag when you next search on your tag groups.
To do this please follow the instructions below:
1. Go into the Settings icon
2. Select the Global Settings tab and here you will see your global tags listed inside the text box.
3. Delete the tag which you have just removed from your contacts so that you no longer have tags that are no longer relevant inside this box.
4. Click on Update Settings to save.