I'm no expert on Shroud stuff, but for the sake of giving you a quick general response, the situation is this:
The area selected from was part of a repair to the Shroud done at the time of the fire right before it was given to the church in Turin.
Apparently this particular part of the repair was not documented. It was an expert job using an invisible weave that so closely resembled the original material that it was not detected until someone used some kind of spectral photography (?term?) a few years ago.
So the scientists, due to no fault of their own, tested the repair, not the original Shroud material.
Hope that helps, and I will try and get a link to better info for you.
The Shroud also happen to be involved in a fire around the time that the Carbon test date it, hence the burns marks on some of the folds, and fire of course has no affect on carbon readings...
"It was an expert job using an invisible weave that so closely resembled the original material..."
Does that make sense to you?
It doesn't to me, given the technology that was around in the 1400's and given the technology that examined the shroud in 1988.
That said, I concur that no one can determine how the image was formed.
The actual shroud would be a revered and honored cloth indeed as would any item associated with the crucifiction. I do not doubt that it would have been kept and protected thru the centuries. Other relics like the titulus crucis were hidden away as was the shroud for long periods. That certainly helped its survival.
So it is not unreasonable for it to have survived.
I just found it strange about testing a patched area....