AC Adapter Input vs Output Power?
I do power supply design for a living. When I specify that this circuit is rated for 19.5v/3.34A 65W, there could be so many reasons for this. Few arbitrary examples include:

-Transformer saturates at 70 VoltAmps (VA)
-The transistors I am using start breaking down after 21V and/or 4 Amps
-My linear voltage regulators start sinking too much heat due to high voltage differentials
-Capacitors start having huge ripple currents that they cannot handle (ESR losses)
-I am unable to meet compliance in powerfactor/emissions at higher power ratings


any many more possible things...

EDIT:
As for the 1.5Amps input current, this is the maximum instantaneous current the adapter will pull from the input. It is NOT an RMS value.