Spam Rating percentage is based on the number of spam complaint responses from constituents that received messages from your site.  We receive spam complaint responses from many of the major email providers, but there are some from which we do not receive spam complaints.  We take the number of complaints that we receive and determine the number of complaints per actual message sent.  We then take that average number and apply it to the remaining domains that do not provide spam response data, and determine the most accurate spam rating possible. 

This is why if you take the exact number of messages sent and the number of spam complaints to produce a percentage, the number in the upper right will be slightly higher.  Spam complaints are not when a constituent unsubscribes or when the email goes into a spam folder ? spam complaints are when the recipient reports the message as spam, and this metric is used to get an overall idea of what percentage of your recipients consider your message spam.