You're almost correct here - if a device advertises a particular power rating, they need to provide the configurations you linked. But they are allowed to provide any arbitrary configuration beyond their marketable power rating. So consider a charger that has a smallish inductor in its DC/DC supply, and a DC/DC chip with an integrated MOSFET limited to say 2.5A switching current. For thermal reasons, that charger can't safely provide more than say 1.7A regardless of voltage. Because it can't supply more than 1.7A at 9V it can't be listed as having a PDP of more than 15W. However, that charger is allowed to provide that same 1.7A at 20V if it wants to, and advertise that configuration to the device - as long as it doesn't list that as its rated power. This sort of charger would be functionally providing >30W, but would only be allowed to put 15W on its packaging/advertising materials. From a user perspective, they see a charger happily providing 30W to one device, plug in another device, and get a lower output power. The lower power is exactly what it says on the packaging (which the user threw out ages ago and never looked at) but the user is still frustrated.