Apparently the manjistha I use, although also know as Indian madder, does not derive its color from alizarin, but from munjistan. (
reference) So I am wondering what other differences there are among the different madders that other soapers may actually be using, since there are really so many species within the genus that are called madder.
Also of interest in that link (which I may have got from you or found on my own; I don't even recall at this point) is that hard water gives better red. Since so many of us have learned to use distilled water in our soap, even if our tap water is hard water, that's not what we're generally using to mix colorants. Adding a single tablet of Tums or some other form of calcium & magnesium salts will apparently bring out the red.