Related: Users in game not showing correctly
I’m pretty sure it has to do with the fact that their presence in game doesn’t update constantly, if they load a different page while in game (or if they have an extension loading stuff in the background) it resets to website.
Edit:What bugs me is that if their presence is set to website, it’ll tell you they’ve left the game regardless if they have or not. (Clicking play on the game’s page will still automatically put you in their server if you’re on their friends list though)