I always wondered why the heck they would pick a portion to test that was clearly from a PATCH
But I thought that I had read somewhere that the patch was an ‘invisible weave’ repair that matched the original almost perfectly.
You answered your own question. The patching technique is French Invisible Reweaving. . . and is meant to match perfectly. The master weaver matches the threads by dying them to match the original and them twists the new threads into the old and then replicates the weave pattern continuing the original. Under a powerful microscope you can see the dye coating and the fixative on the cotton threads that is simply not present on the original FLAX threads of the Shroud. Chemically they find that ALUM was used in the retting of the cotton that was not used in the retting of the flax. Different techniques were used on the bleaching of the two different age threads.