I have experienced a weird situation with [Whatsapp](https://play.google.com/store/apps/details?id=com.whatsapp), and I don't know what it means. 
I sent a message and it got the blue double checks at 18:00 hrs, but the person to whom I sent the message was last seen at 16:00 hrs.

How could my message got  blue double checks if the person hasn't opened **Whatsapp**?