Perhaps this was already sorted out, and if so I apologize. But...

I was looking at the color indicators on the character pages, and wondered why several characters who were attacked (and likely killed) by walkers are highlighted red (dead) instead of grey (undead).

We know that bicycle girl was Hannah from the first webisodes series. Though we see her being devoured by a group of walkers, we see that the result of that is that what is left of her corpse reanimated and reached for Rick when he takes the bike in Season 1. Even though we don't see Hannah's reanimation, we know it happens.

So, for example, why is Sean, a member of Dave and Tony's group highlighted red? Yes we know he was killed by zombies, but given the evidence of bicycle girl, what is left of him will reanimate. So, why not show his status as undead instead of dead?

In summary:

The only way a person that is devored will not ranimate is if the zombies happen to bust open their skull (or the victim does it themselves during the struggle). Even if the head is severed from the body, the head itself will reanimate. So, I am proposing that all characters in TWD universe that we see attacked by zombies be given a status of undead instead of dead, unless we know that they are dispatched by some type of head trauma. (Tomas for example is clearly dead)