Microsoft had already begun work on Windows 8 before Windows 7 was completed. The reason being that different teams work on different aspects of the operating system, which is why Microsoft was able to do this. Microsoft previously shed some light on what we can see in Windows 8 through a job description.
It appears though, that one specific Microsoft employee was not too careful with their LinkedIn profile, and had accidentally leaked some interesting details regarding Windows 8. According to his LinkedIn profile, which seems to have had many of its details removed, Robert Murdon has been working at Microsoft since January 2002. Windows 8 News managed to catch this on the Microsoft employee’s LinkedIn profile before it disappeared.