With Soapbox Petitions, individuals are allowed to sign a petition more than once. Should they do this, all signatures of theirs will be included in the total number of supporters of that petition in Soapbox.
When an individual signs your petition, a Lead record is created in Salesforce and associated as a Campaign Member of the Campaign you selected for your Soapbox Petition. Individual Leads will be created for each signature, whether or not one individual has signed the petition previously. Once those Leads are converted, however, if multiple Leads are matched with an existing Contact, that Contact can only be associated as a Campaign Member of a given Campaign once. As an example, if one individual signs a petition ten times and all ten Leads are converted to match with the same Contact, that will mean that the ten signatures tallied in Soapbox are now represented by only one Campaign Member in Salesforce.
The difference between these two totals represents the total number of signatures in Soapbox Petitions as opposed to the total number of unique individuals signing the petition in Salesforce.