I'm up to S3E03, of the walking dead, so I understand that everyone is infected, and when a person dies of any cause, it takes a time frame of 8 minutes to 8 hours for the dead person to rise back as a walker. What I don't understand is that they are only meant to come back after a certain time period, but when Rick for example ends up in that tank in the city (S1E01), there is a dead soldier who looked like he's been there for days, and way longer than 8 hours, but when Rick sits next to him in exhaustion, the dead guy wakes up. Why and how??? Another example is on the highway when Rick and the group are travelling, they break down, and corpses that have been there for weeks suddenly wake up. Some have actually woken up when they were supposed to, like Randal in S2E12 when Shane snaps his neck in the forest.
Why do some walkers wake up in the presence of a human and some wake up where the plot of the story has said they wake up?