6V is the real-life voltage once you apply a load, there is no point in testing 20V, 40V, 60V which indicates there is no load attached.

As the pdf shows, the testing begins at 500Hz. This value was chosen because the The Plug 3 cycles off at 250Hz. The D1 will work at a lower value but I wanted to keep the test equal for all devices if possible.

In other words, this test is not a quantative test of a converters output at a given speed, but rather when there is enough power supplied to provide a 500mA output, what is the efficiency transfer of the converter.