You do realize that the picture you provided isn't of a hog's anatomy, just a skeleton. It doesn't actually show the base of the brain stem that you say you can see. It doesn't show the location of the external pinnae which are what most folks refer to as ears. It does show the bony external auditory meatus, but few folks would recognize that for what it is and it isn't labelled in the drawing.
In invite you to read my first sentence again since you obviously missed it.
If you treat everything as "ear" from the ear's bony structures and all the way up to the tips of the external pinnae that most folks call "ears," then allow for a lot of head tipping, directly behind the ear ends up involving a good bit of neck tissue and some air space above the neck.
Given that the bony ear structures shown in your linked skeleton are at the base and below the external pinnae, most folks note shooting relative to the base of the ear, such as "directly behind the base of the ear."
In your image, directly behind the ear with the hog's head at that angle would produce a shot that went into the occipital of the skull, at least for the 1-1.5" behind the skull as shown. Beyond that distance you would come into the atlas.
As rickyrick said, the hydrostatic shock will do its thing nicely, even when you don't actually hit the brain stem.