Why Is Recurly Syncing Invoices That Were Created Before the Set Start Sync Date? (QBO)

When this happens, it is because the invoices in question were either unpaid or past due on the sync start date.

In this scenario, Recurly will sync these invoices because it needs an invoice to attach the transaction to once the payment is made. Therefore, it is expected behavior to see invoices created before your sync start date if they are still unpaid or have a past due status.