It could have been on top of the body, but with the rifle/hands being moved it became dislodged.
The police might have even moved it to get a better photo of the body.
If the bible was on top of Sheila's body, that just makes Bamber's guilt even more established.
There is no chance of the bible landing on top of Sheila after she killed herself.
But as said, no police have said they moved the bible prior to the photographer arriving.