User blog comment:Callofduty4/Call of Duty: Ghosts: An early look/@comment-4578721-20130608185416/@comment-1646923-20130608190333

CoD has 4 separate timelines (universes) which are:

The Modern Warfare timeline: The Black Ops timeline: The World War 2 timeline: The Zombies timeline: The Ghosts timeline:

So it doesn't seem too weird to me