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I think I can help here.

I did my master's degree research on biofilm that builds up in water lines.
Let me explain this simply.
Over the centuries, the Shroud has undergone some decay, just like any other piece of cloth that you might find from that era, only more limited due to care and preservation efforts.
Bacteria have been breaking down the cloth since it was woven into the Shroud. Now, when bacteria break down some of the cloth, divide, and die, do you think they just fall off the cloth? Well, some do. Many, however, do not. They form a microscopic layer around the thread. New bacteria are constantly growing on the cloth, and they colonize on the layer of bacteria which has been breaking down the cloth.
Multiply this times many centuries. Now, envision a strand of thread surrounded by many layers of a substance that is almost like plastic or scotch guard. A cross section would look something like the rings of a tree. The center layer would be the actual thread, while the surrounding layers would be dead and living bacteria.
Carbon dating measures C14. Since the decay of carbon 14 is constant and known, one can measure it and thus determine how long it has been decaying. This tells the researcher the age of the item being dated.
Now, is it only the actual cloth that is being carbon dated? No. It is the thread, plus all of the layers of bacteria, dating back from the cloth's origin to the present. One layer may be 2000 years old. One layer may be 10 years old. One layer may be 5 years old.
Thus, the initial carbon date obtained in the 1980s did not determine the age of just the Shroud. It determined the average age of the thread combined with the many layers of bacteria surrounding it. If it had determined that the age of the Shroud was 2000 years, we could thus assume that the Shroud is actually much older. If, however, all of the bacterial layers are removed, we are testing only the actual thread that forms the cloth. Thus, the new reading should be quite a bit more accurate.

I hope this helps. If you have any questions about this, I might be able to answer them a little more thoroughly.

The shroud was also damaged and nearly destroyed by fire and that itself would skew carbon dating results. It's my understanding that any addition of more recent carbon particles to a sample would have that effect.

More convincing was positive identification of plant spores that only exist in the Middle East and the fact that the "thickness" of the image is literally only a few molecules. There was some deposition of ochre, a pigment, but most of the image was apparently caused by some sort of effect that altered the color of a few milimeters of the surface of the fibers. It's logical to conclude that the ochre was added sometime in history by someone trying to "touch-up" the image.

All in all, the conclusion that the carbon 14 testing proved the shroud to be a forgery was shaky at best. When it came out, however, the press leaped on it as "conclusive" without any reference to a hundred other indications that it could in fact be the burial shroud of Christ.