*delivered* indicator shows up late

So heres what happened:

Sent a message on Saturday, check on Sunday, does not say delivered. Call the contact, straight to voicemail. Receive an unrelated text message from said contact on Sunday. Respond to text, does not say delivered for several days( checked on Wednesday). And as of today (Thursday) the messages i sent on Saturday and Sunday show as *delivered*. This has happened several times to me in the past, what does this mean? Thanks…