I sent somebody a message on Facebook, and their active timestamp now reads "active 1 hour ago", but the message says that it has been seen, even though I sent it less than an hour ago. It didn't say 'seen' as soon as I sent it, but rather about 20 minutes later, but according to the active time stamp the person was not active when the message was seen.
The reading on the browser and on the messenger app agree with each other in both respects.
How does this happen?