Genghis is right, it depends on the choice of reference for the aoa gauge.
PDR1 is right, the choice is biased towards something easy to set up and check. Fuselage centreline datum is often chosen
But to get to wing aoa you have to add the wing/body setting which is usually chosen to get a more or less level cabin floor in cruise.
On top of that, the actual average wing aoa will depend on the design twist as modified by aeroelastic twist under load.