While a PhD is short for a Doctor of Philosophy, at universities such as Princeton or Stanford the title "Dr." is reserved for medical doctors. At less secure schools one sees the title imposed by administrators. It's a wonderful poker tell; you'd think people would be aware what they're signalling? Or they know, but we're not their intended audience.

In Germany, "Herr Professor Doktor" is often in jest, as one usually restricts to the highest ranking title. And titles are fading, a form of class-consciousness. To use "Doktor" unnecessarily reveals one is not a Professor.

So I don't understand the cultural context for this author's use of "Dr." It's welded to his web site dr-axel.de; perhaps this is a stage name like "Dr. Ruth".