Are you being serious? The chart definitely has science behind it (probably classified under thermodynamics, but it's such a basic concept, it might be considered "general science), and that science is closely related to dew point, and is easily observable in your home.
The fact is, the surface temperature near your walls and windows (notably the latter) is measurably cooler than the temp in your house (no matter how good your windows are). While the provided chart is obviously an approximation of an average home, it is not without scientific base, as you claim. You might've learned about dew point in grade school; not why it is (these concepts being beyond an average gr. 7 student's comprehension), but how it's calculated using pre-established psychrometric charts. Those charts are created using various types of science, but it is pretty observable with basic tests (which is why it's done in grade school, at least mine, and my school was quite basic).
Maybe in your specific home, when it's -20°C outside, you can get away with 28% RH, but it's close enough that general variances (airflow namely, but also your guess of the outdoor temp. as well, among other factors) will make the chart "accurate enough". Much like an "ideal weight" chart, which is also very obviously not useless (note: it's an ideal weight range chart, and comes with the caveat that you're an average person, not a peak-performance athlete, or a sedentary quadriplegic ... both of which would obviously have "side-case" ideals).
Sounds like you don't understand the concept of these recommendations being representative of an average situation. Most people have a hard time doing their taxes (which is adding and subtracting), so having an equation on their humidistat with 25 variables that they have to solve for to know where to set their humidity at? Obviously unrealistic.
One who is offended by truth, has no place among those who seek wisdom.