I save my strings and trimmings for recycling, so it occurred to me that I probably still had the trimmings from the last set of Martin SP that was on the guitar, for comparison. I dug through the box and pulled out all the .053/.054 trimmings, and this is the closest match:
The one on the bottom is the current set. I colored the end of it with blue marker to keep from getting mixed up.
So the previous Martin set was also wound a little shorter than the others, and I didn't remember it; but there was more exposed core and less winding on this set than on the previous:
Sorry that's a little blurry. Camera didn't want to focus. The difference in exposed core length is about 1/2", with more exposed core on the current testing set. However, the crimp where the string was chucked into the winding machine is about the same distance from the end in each sample.
In the initial notes I said this: "The wrap wire is a tiny bit uneven. In directional light, I can see highlights, little sparkles, where the wrap wire is not completely smooth, and those areas are faintly tactile under the fingers." This photo shows that:
Again, the top one is the older sample, and the wrap looks much more even, whereas the current testing sample is less even, showing several bright highlights, which are tactile as well as visible.
I no longer doubt these are genuine Martin, but I think it's clear they came from two different production machines, or different production runs.
So: there are three possibilities. 1) Production variations can matter enough to make samples test differently. In that case, the test is of limited value where Martin is concerned, and any other manufacturer that allows this much variation. 2) The production variations here are not the cause of the variation in results; rather, it's due to the inherent subjectivity of this test, in which case the test is of limited value generally. 3) An unknown third factor is at work, perhaps environmental, like humidity. It's worth noting that I fixed a problem with my whole-house humidifiers after this test was underway, and the hygrometer inside the case reads higher as a result.
This is discouraging. I wanted this test to be more repeatable.