Good, now that we are all on the same page, let me lay out my reasoning on why fast and slow zombies can coexist in the zombie genre in general, and in my zombie world specifically. Mind you this all only in regards to the undead variety of zombies; even if some of the reasoning about speed and strength can be applied to the "rager" types.
The Pathology Of the Zombie Cycle
STAGE 1 (Infection and 1st Death): A person is infected. Through whatever means a person "catches" whatever it is that will eventually re-animate them. They are still alive, but usually just barely. Eventually, the person dies; brain functions cease and the body starts to decay immediately. The brain is the last to go, as it slowly dies from lack of oxygen. Even if resuscitated, higher brain function can be irreparably damaged.
STAGE 2 (Re-Animation): The dead body is re-animated, usually within hours, if not minutes. The reanimation agent, usually described as a virus, reanimates the brain first. Higher functions are gone; due to brain death. Reasoning, communication, and all the other skills that separate us from the animals are gone. Base instinctual functions, such as locomotion, hunger, etc.are all that remain. The body is clumsy and slow to respond; this accounts for the Romero-esque view of the newly undead shambling zombie. They can attack quickly from short distance, usually from behind it seems, but can't chase prey down if they run away.
STAGE 3 (Rigor): For whatever reason decay is slowed down to a snails pace. How else can we explain zombies hanging around, rotting, for years? Rigor mortis will eventually set in. The process can start before re-animation, depending on how long Stage 2 takes to manifest itself. This accounts for the stiff legged, stiff armed zombies we see. Eventually, rigor mortis will go away naturally. How long this takes, seems to vary from person to person.
STAGE 4 (Evolution): After rigor mortis goes away, and sometimes before, the zombie "evolves", for lack of a better word. Limbs become more pliable and the body's natural healing processes take over, as best they can. The brain begins to re-route signals and gain more control over the body. Coordination increases and the instinctual processes become more focused. This accounts for two types of zombies we see:
- Fast/Strong Zombies - Some zombies will still suffer from rigor, or injuries sustain at or after death. They may be slow, but the pack mentality of the horde manifests itself. Others will not suffer from things that could slow them down. The bodies can move, almost as if they were still alive. Because they are dead, though, they have a distinct advantage over their human prey; they do not suffer from the regulation of pain. As a result, the zombies who may have been average in life are super fast, or super strong, or both in death. Even though they may be more dangerous, these types of Zeds tend to fall by the wayside quicker than the shambler type, as tendons are pulled, and muscles and ligaments are torn, as the bodies are pushed beyond all tolerances in the pursuit of prey. Eventually, the bodies are unable to function; physically destroyed, because of the abuse.
- Intelligent Zombies: These are rare, but they do pop up from time to time. These are Zeds whose brains were not damaged as badly as the normal type of zombie's is. As the body tries to heal itself, some higher reasoning is restored. These types of Zeds are extremely dangerous, because they can reason out the hunt and see potential dangers in their environment. On rare occasion some have an intellect on par with the living; and even higher at times, as in the "Monster Island" books.
STAGE 5 (Decay and 2nd Death): Despite the fact that the body will try to heal itself, it is dead. Because of that, it is a losing battle. Barring being killed by the living, or falling prey to environmental dangers, eventually, the body will decay to the point that it is nothing more than a pile of bones and ooze. Remember the danger posed by these types of Zeds in the first person accounts in "World War Z"? This decay may take years, but can apparently happen in days/weeks/months, as seen in "The Walking Dead". Eventually, the body is unable to sustain any type of activity, and the brain rots away to nothing, bringing about a 2nd and final death.
There you have it, my explanation as to how the fast and slow zombie crowds can live in harmony, without completely destroying canon in any of the zombie worlds pop culture has brought us. Now back to gaming the Zombie Apocalypse.