Do you see
where the stamp is placed on the back? Right over the flap of the envelope. This gives added security. It's more common than not to find Indian region covers with the stamps on the back. Here are a couple taken at random from my Cochin collection, because I happened to have my Cochin Photobucket file open:

and

When you
do find the stamps on the front, it's often a postal stationery envelope, leading the sender to stick any extra stamp(s) alongside the indicium:
