What tone to use in the personal statement?
I guess that the guide encourages applicants to not hide behind a neutral tone. You should "be yourself" and write in the way you normally talk to people in the workplace. 

You may consider reading your personal statement (or asking someone to read it), and then try to assess the tone and imagine how you could describe a person who wrote it. Can you say something about the author which is not already explicitly mentioned in the text? If you can not see a specific tone and can not guess anything beyond what is already written, then the personal statement is probably too neutral.