Max and/or avg swing per hour

[Angel]

Whenever I have seen a swing per hour it is always described as +/- xBB. Is there some reason these numbers should be symetrical? If after 100 sessions, your greatest loss in a $20/$40 game is $1000 but your largest win is $4000 how would one determine max swing? avg swing?

Thank you for any help you can give me on this.