Only contacts who have been sent an email (not including internal emails) in your workflow and later go on to meet the goal criteria will be counted toward the conversion rate on the Performance tab.
The goal conversion rate and number includes contacts who complete the workflow after having been sent an email in the workflow and later meet the goal criteria, even though they are no longer in the workflow.
If a contact meets the goal criteria while still active in the workflow, but they haven't been sent an email from the workflow, they will still be unenrolled from the workflow, but they won't count toward this number. To be clear, contacts who were sent an email from the workflow only refers to email actions where the email is sent to the contact, not internal emails sent via internal email actions.
If a contact is suppressed from the workflow as a result of joining a supression list or if a contact is unenrolled from the worfklow after being enrolled in another workflow, and they later meet the goal criteria, they will not be counted towards the goal criteria number.