And the seemingly obvious answer to that is that the spell did rebound, in a fashion, as it killed a part of Voldermort’s soul (apparently the Killing Curse can kill horcruxes). And that segment of soul was more than an invader, a parasite. It was a segment of Harry himself. As such, as the spell hit, it affected both of them. Therefore, while Harry was protected, he was not shielded, and therefore travelled to ‘King’s Cross’.
Forgive me for not understanding, but surely both Harry and the horcrux were protected by the charm? Neither the Horcrux, nor Voldemort himself, should have been able to die, seeing as they both should have been under the protection of Lily’s Charm, as Dumbledore reveals. (I would have said that the charm couldn’t work on Voldemort, who was so far removed from Love, but I am clearly wrong with that!) And whatever caused the spell to rebound should have done so here as well! (And there was no scar here, although in Godrics Hollow there was!)