What ballistics tests show that such a round will deflect sharply upwards upon encountering one layer of skull bone? Here is the Tracking Oswald part 5 ballistics dummy test:
The round didn't deflect sharply upwards upon encountering one layer of skull bone. It "deflected" upward after encountering a layer of skull bone. The encounter with the skull bone had deformed it to such a degree that upon entering the brain it developed a lift that curved its trajectory upward.
What? No, fragments deposited at the base of the head could not have "leaked out" from a hole in the parietal bone.
Why not? Fragments would have also exited along with brain matter at the time of the head "explosion"
Because severe damage to the cerebellum would be an inevitable obstacle for a straight-moving bullet. Do you think a 6.5 round deflected sharply upwards as soon as it got past a single layer of skull?
When it passed through the skull, the bullet was no longer moving in a straight line trajectory. The cerebellum was far enough below the point of entry to escape extensive damage from the deformed bullet as it passed above on its curved trajectory.