Hmm.
I never tested a stalk, or even stem, for that matter, so ... no data for you there. I'd be naturally suspicious.
I can say that I frequently tested wads numerous times, rerolling and re-squishing the same wad, and I would find that the reading was consistently higher on each successive squish. It was never lower. So I was left wondering what a true result should be. The first one? The last one?
Some would stay the same on the 2nd squish, most would rise a point or two. I reasoned that each squish had less fluid so the ratio of solids would naturally rise, and so the best measure would be when the solid/fluid ratio was fresh and undisturbed. I never did compare to a mortar/pestle process which would be the best standard, but that's just a lot of extra work.
So, it's back to whatever standard you use to test. In this case, it's from a part of that plant that had been severed for ... awhile? ... how long? It could be that, it could be a reaction to the trauma? And I've tested petioles, but not stems/stalks, so I wouldn't know what to expect. I think I remember that the reading you will get rises as the severed part ages over 10-15 minutes. I was careful not to let a sample sit there without testing it.