The spell in Godric’s Hollow rebounded, as is repeatedly affirmed. Perhaps a better question would be why that didn’t happen here.
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’.
This still falls into the ‘flights of fancy’ category as expounded above.