This is likely due to a scrubbing or cleansing issue that was performed by your Mailer. Unfortunately, we don't have a way to monitor how Mailers de-dupe their mailing list against your Suppression List outside of our system. We always recommend that your Mailers use our cleansing tool to help mitigate these types of potential issues.
To learn more about Complaints, please review this article. (placeholder for link to article)
If you have further questions, please contact us: firstname.lastname@example.org