When a Registration Lead record is converted to an Opportunity (aka Donation) record, your Soapbox Events for Salesforce packages add a Contact Role record in the Contact Roles related list for the registrant and its Primary checkbox is checked.
Depending on your NPSP Settings, the Primary Contact lookup field on the Opportunity may also be populated. This is not done by your Soapbox Events for Salesforce packages but rather the Nonprofit Success Pack itself. The Contact used to populate this lookup for Household Accounts is the Primary Contact from the Household Account. In many cases, this will be the same individual as the registrant. In some, it will not.
If you wish, you may manually update the Primary Contact lookup field to match the Contact with the Contact Role marked as Primary on the Opportunity.