Why is it that sometimes on Facebook it shows the chat ticker thing at the bottom...?

[MissMegz]

...and sometimes it doesn't? And then when you go over to your info or
something it just pops up. That happened to me today, and I had a missed
message left there, but the chat screen never popped up when the message
was actually sent... Now I feel bad for not answering. :\ Lol. Does anyone know
why this happens?