You forgot to factor the voltage drop under load.It is a big factor in entire PSU efficiency, since dc-dc is only half of PSU a PC would need.

If it is 12v dc-dc PSU, it passes the 12v along with minimal losses

If it is 19v ac-dc high efficiency, a 19v -> 12v dc-dc is needed and that takes a chunk out of the efficiency.

For example:

19v AC-DC 94% efficiency

19v->12v DC-DC 95% efficiency

Both very efficient, high quality - 94-95% is spectacular.

But combine them together... 94%*95% gives overall efficiency around 89%. Good but no longer impressive. A 91% 12v AC-DC + DC-DC may not look as impressive on paper but would come out a little more efficient since the DC-DC only needs to do conversions for minor rails like 5v, 3.3v, etc

12v, as you stated, is optimal if there's no resistance, impedance and inductance in the circuit, which is the exact oposite of what happens to be in a computer board.

19v normally is the way to go of most manufacturers because of that. if there's a verifiable current drop (on heavy loads there'll always be), it'll still leave the voltage above the 12v needed for the computer to work properly.