This was an issue in Insights as well, default functionality of excluding ticket tags doesn't work, and the number shown increases.
Workaround in insights was to create a custom metric which you would use to filter, however i haven't gotten this to work in Explore. I also see on the community forums others are unable to get this to work. Including ticket tags works, but not excluding.
We need the option to easily exclude ticket tags by adding the filter, choosing exclude and selecting the tags. (This is already here, just doesn't work!).