How do power adapters automatically detect the output voltage?
Technically speaking, there is a mechanism how laptops identify capabilities of power supplies and act accordingly. For example, DELL laptops will refuse to take power supplies made by HP, and vice versa. This is done through a proprietary serial protocol that goes through the center pin and gets information from specialized secure serial EPROM. So it can be imagined that an intelligent PS can decode the serial requests, determine the kind of laptop, and set proper output voltage.

But I would consider this as a very remote possibility, and the label on your PS simply indicates that at 18V it can supply somewhat more current than at 19.5V, creating a confusion. I will be glad and surprised if it proves otherwise.