If multiple emails are sent to the Service Desk maileater email account within a very short time period, tickets may not be created based on the order that they are received or on the sent date/time. Therefore, the order of reference number of the created requests may differ from the order of the date/time that the emails were sent or received into the maileater mailbox.
This is working as designed. The emails are processed in parallel by Service Desk. There is no way to guarantee that tickets processed by maileater will be created in any specific order.