Why does Lenovo A316i has two different back side?

Actually, the phones are slightly different models.
On the left, with the camera in the corner is Lenovo A369i announced Sep, 2013
On the right is the Lenovo A316i announced April 2014
There's a slight overal size difference, slight weight difference, battery difference, cost difference, but many specifications are the same. The camera and microphone were moved.