I dunno specifically, but the dynamos put out AC so it's somewhat more complicated than DC. Depending on your meter and if the load, you may be reading a no-load peak voltage rather than running RMS voltage. It is possible that the dynamo has a protective Zener that will short it out above some level, like 15V but that would be wasteful of you power if you reached that level in use. If the headlight is LED, it almost certainly has its own regulator and protection, but again I don't know specifically.