Why did they release ipad 4 same year as they release the ipad 3?

The ipad 3 was release in march 2012 and ipad 4 was announce in october 2012 and ipad 4 is gonna be in stores on November 2 2012 why did launch ipad 4 7 months after the ipad 3?
>>> Why did they release ipad 4 same year as they release the ipad 3?