One of the many mysteries surrounding his survival of the curse.
Harry is the only person to ever survive the Killing Curse, which usually leaves no marks at all, so it is a mystery as to why it marked him in such a specific way.
Perhaps it was the result of his becoming a horcrux?
Perhaps it reflects the pattern in which the spell exited his body when it rebounded?
Perhaps that's just what happens when one somehow survives the Killing Curse?