Since there's no obvious damage other than a few scuff areas in the bore and the ring seams to fit fine down where heat doesnt get to, the probable cause for all the compression loss is the blackend area around the exhaust port. That area might have warped out from running hot. We don't know if DL runs their parts through a heat soak process before honing the bores. So it's very possible the cylinder warped out of shape at some time. Run another 10 or 20 gallons through it and maybe it will get better. Can't hurt.
PS. The Amsoil appears to have protected the parts quite nicely. Run some SeaFoam in your gas mix and maybe it will clear up the Carbon. Again can't hurt.