Why does it show that a person read a message on fb, but really didnt?

So think of it like a text message.
This has happened to me:

I'm texting with someone, and I put my phone on the counter without closing out of the conversarion screen.
I turn away to do something and I end up doing something else. Then maybe something else.

Now, because I didn't close out of the text screen, when the other person replied, it didn't send an audible notification, and I havent looked at it, but on the other end it shows as 'read'

Then the phone screen 'times out' and closes on its own, and I don't actually see the text message until I open it up again.

This type of thing has happened on FB as well....so there's that possibility

Or, they just didn't feel like responding at that time and then lied. 😣