RE: What Happened?
You mention a "narrow shooting lane" that you shot through. Is it at all possible you may have nipped a small branch that you didn't notice? This could explain you not getting a pass through considering where you describe him being hit. The area you think you hit (how you described it) should have resulted in some kind of a pass through I would think.
I can't imagine anything in that part of a deer's body (low and back from the heart)that would have stopped an arrow from passing through.