It doesn't show any blood trail from the neck down to the back. That " pattern " of blood-staining is also on the sheet of the bed too.
No mystery !!
Eh? Sorry, have I missed something? I understand from having seen this diagram before that it is Sheila's nightdress being analysed? You can see blood trailing from Sheila's neck can't you? This made the stain around the neck on the back of her nightdress!

And the lower stain was formed by blood under her arm trailing to the back of her nightdress, it's called gravity!

Are you suggesting this is June's nightdress?