This study was never reproduced and as far as I know it was never accurate in the first place. Every reference I've ever heard that was made to this study was certain to point out how it was not an example of scientific evidence for the soul...
It could be very probably electromagnetic activity in the brain. once the body is dead and that activity is stopped, that energy is well, gone. theres so much we dont know about in the brain, that im sure loss of weight through death is that sort of activity.
This theory isn't true. The only weight that would change would be from blood/bodily waste leaving the body.