I took some pics yesterday that may make things clear.
This first pic shows the gap between two new Elring head bolts. BTW, I notice these are all Class 12.9, so they are extremely strong.
Held together at one end (with ends flipped so the heads don't get in the way), these new bolts have the same thread pitch so they fit almost perfectly and the light gap coming through is small and uniform. Any variance is likely to be due to the perspective of the camera.
Second pic should be like the first one except the bottom bolt is a new one, and the top bolt is used (original, came off my M54). The threads at the left end are held together, but the top (old) bolt is stretched causing the gap to grow along the length of the bolt. By the time you're at the far right end of the photo (BTW the old bolts had a bigger shoulder section on them), I'll estimate this mismatch has grown to about 1/3 of a full thread pitch, so call that about 0.5 mm.
Trying to measure that using a caliper might be tough to do, but lining up old and new like this makes it easy to see.
Close up of the left section (new bolt on bottom, old one on top)
Close up of the right section (new bolt on bottom, old one on top)
And again, in case it was not clear earlier, the stretch of the old bolts appears to be pretty uniform. So if I held the right side together in the above photos, you'd see a mismatch on the left end.
And BTW, for those Physicists on the list, this seems like interferometry.