Delivery receipts are information packets that have been generated by the handset. If the handset is not equipped to show the message, but has received the signal from the sender, an issue can arise where you can receive a successful delivery receipt, but it has not been fully received.
In this case your recipient may never have seen the message. This is particularly common with older handsets where the memory is not enough to display the message in its entirety. If you believe this is happening contact Sinch Support who we will investigate the issue further for you.