Are you using Chrome on your main computer at home, and are you logged in? It has nothing to do with IMDB, it’s your browser recognizing that you’ve been to those URLs. I suspect your Chromebook and your Android phone all log you into Chrome which is sharing information via the Google servers.
Yeah, Chrome keeps all that stuff synchronized. So does Firefox if you use Firefox Sync. If, at home, you aren’t using Chrome or aren’t logged in, it can’t synchronize.
Other versions of VBulletin work regardless of what device you used them on. They track the user account at the server. This version of VBulletin just doesn’t work right. It can’t remember if you use multiple clients, and it just fails in general most of the time. Clicking “take me to last post” takes me to random posts all the time. It’s unfortunate because a working feature would be really nice, but we’ll never see any upgrades on the board software.
How are you seeing that your main computer doesn’t know the last posts? Is it simply that the threads display in a different color or is the “New Posts” acting differently?