We are assuming that Harry is a horcrux in the traditional sense or any of that. Yes,creating a traditional horcrux requires the spell and the murder, but things were already different there.
1) the requirement for murder was more than satisfied when he killed Lilly and James. I think that fills the quota for murders to make a Horcrux
2) the protection on Harry made something happen that wasn't supposed to. Any number of things could have happened in the time the spell connected and Voldermort was shoved out of his body. For one thing, with his soul already split, the loose part of his soul could have passed into harry through the spell that backfired. No one knows what happens when Aveda Kadaveda fails,it's never happened before.
3) Harry is probably not a horcrux in the traditional sense; think of the powers he has and everything he got from Voldermort. How did it get in Harry? It got in through the spell that failed, therfore, it might have come attached to something like a chunk of Voldermort's soul. See point 1, the murder requirement was met therefore the spare bit of soul was free to move along any magical conduit attached to voldermort at the time; so when he lost control of his power, he lost control of where the soul moved and it passed into poor Harry, giving him power and sealing his fate
Harry no unmei... fuuin! (destiny of harry...sealed!)
NOW.. THIS MIRROR SIRIUS LEFT HARRY MAY BE A HORCRUX.. by the way she speaks [or someone speaks for her =)] its seems that the mirror is really important and it didn't appear in book 6, so it has to do in the 7.. just guessing it might have that purpose.
Or worse, Harry looks into it during the final fight (or before) and realizes the scar on his forehead means he;s the horcrux