Thats because your computer is likely set up for Microsoft update rather than plain windows updates.
While Windows Update provides you with updates specifically for Windows, Microsoft Update expands the service to download and install updates for other Microsoft software, such as Microsoft Office and Windows Live. Automatic updating is a feature that allows you to set your PC to automatically download and install updates using either service, making it easy and convenient for you to keep your Windows PC current.
You can turn off office update listings by clicking on "Change settings" in the left panel on the Windows update page, then unchecking the box under the heading "Microsoft updates". Alternatively, you can just right click on the offered office update and hide it so it wont show up in the future.