What happened if I sent a FB message they never 'seen'?

[Malevolence]

I have a friend on FB who most of the time views FB through their mobile phone and when I finally see they were online (green spot in chat, not the phone logo), I sent them a message. I left the chat open for 2 hours and I noticed they never 'seen' my message, since FB lets you know if they saw it at a certain time or not in the chat. I just see my message and how long ago it was sent, nothing else. Then they finally change their status to the mobile phone logo again.

So what just happened there, did they ever see my message or even know I sent one to them? Appreciate the help.
Ive sent other messages before and they havent checked those either or i dont know if they ever did. And no, its not because they're ignoring me or things like that - theres no reason they're like that with me.