Ahhh. Well that's probably it then, because I just tried moving the sliders again and that is the only thing left when I move them to encompass the "anomaly" in the histogram.
So in my mind, the only mystery left is why some subs work and some don't. Ordinarily, I wouldn't mind so much. Image History doesn't matter that much to me, but I think it also has an effect on the "Grade Images" tool, which I really like using to help me find bad images. When I run "Grade Images" on all my subs, it always fails to grade anything for those subs that it doesn't like. Must be using the same code for star detection.
Check this out... Looks exactly like the Image History from above...
Well, OK... not EXACTLY like the Image History, because the HFR and star counts are different from the history vs. the Image Grader. But the lack of data for those subs is the same for both.