When I log in, it shows me players whose activity is "disconnected" on my list. Those players should just not show up on the list.
I've noticed this as well, and been very confused by it. I wasn't sure if it was showing people who only recently disconnected or something similar, but it's still a bit perplexing, especially if your friend list grows quite long.
The current bahaviour is that it shows people who are not currently connected, but still have a game running. I agree that should probably change.