In the activity log, there is a Data Value Change activity for Unsubscribed from false to true. After I double click the activity, the detail page of the activity opens up. In it, for the Reason of this change, it has, "System flow action sysActionChangeDataValue". What does this mean ?
This is the result of an automated process known as Durable Unsubscribe. Marketo changed this record when it detected the same change within another record (duplicate) using the same email address. The system will not all one unsubscribed record and the rest of the duplicate records using the same email address set to subscribed. Marketo will make the same adjustment to the rest of the records that are using the same email address as the first record it finds with that email address. This change can go either way From True to False or False to True. For more info, please see this article.