I think the straps are a feature of the latest mark of life preserver. The updated jacket was introduced to hold the head higher out of the water to reduce the likelihood of drowning if unconscious, and the straps must be to stop the jacket riding up (or the body dropping down, whichever way you want to look at it). I heard that the larger size of the stole gave some aircrew difficulties with head movement so they were allowed to stay with the previous mark of jacket. If you notice, OC IX Sqn doesn't have the straps and his stole cover is of different material. As so often, fix one problem, introduce another...