The simple answer is: because everyone with >= 300 reputation can create tags by [just specifying them with her question][1]:

> Simply enter a new tag with your question and it will be created.

Somebody chose to do so and others tagged their questions with the same tag afterwards.

Tag cleanups are performed on an irregular basis by the SF regulars and moderators by means of manually re-tagging all questions using a "bad" tag and deleting it afterwards. If you want the [tag:dell-r710] to disappear, just post a new question making your case.


  [1]: https://serverfault.com/privileges/create-tags