"Well, they did say in the News Report movie that people's changing speed depended on how close the bite was to a major blood vessel. If, say, the zombie bit hard enough to open an arterie, he would have gone pretty fast anyway."
Yeah,and Andy also said that he was losing plenty of blood.

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Or Andy died from the bite due to blood loss or whatever and we all know that people who dies that have been infected becomes Zombies only a few sec after death. Just as it was in Dawn of the Dead original and many other Zombie flicks.

Well, they did say in the News Report movie that people's changing speed depended on how close the bite was to a major blood vessel. If, say, the zombie bit hard enough to open an arterie, he would have gone pretty fast anyway.
Confessions of a Dangerous Fundamentalist.

Yeah that would make sense I guess.

Slight Spoiler
It might stand to reason that in his horribly weak condition that he might change faster than someone else with his immune system being weaker than the well-fed people. Or maybe he was so close to dying from malnourishment that the bite only slightly sped up what was coming anyway.

In "Andy's Last Days" it appears that he passes out in front of the camera, then when he "awakens" he is zombiefied. He probably went up on the roof immediatly after turning out of instinct (same reason the zombies were coming to the mall). It was something he was used to doing. Of course, he couldn't write coherent words on the board, so he just spread his blood on it.
"My give-a-d*mn's busted"
Save a horse. Ride a cowboy.

yeah, Andy goes up on the roof afterwards. It's unclear whether he changes up there in the movie, so one has to leave this to the "instinctual" explanation, same as why the zombies head for the mall.

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