Wait. which display has the MenuBar subtracted from the height of the picture area when you do this experiment?
Half-inch tall menubar at 220 pixels/inch would take away around 110-ish pixels from the 2880 picture area, leaving 2770. No, that is not enough difference to explain this.
It sounds like you are converging on, "my built-in is not showing as large as it says it should be and the specs say it should be." And displays preferences supports the assertion that the built-in is WAY smaller by showing a graphic way smaller than expected.
I have never seen anything like this before.
The resolution of the built-in is not changeable by mere mortals, unless you are using advanced tools like SwitchResX or similar. The resolution of the built-in depends on a resolution stored internally in the Operating System -- unlike regular external displays, the resolution for the built-in is not derived on-the-fly from the display.
But your complaint is even more subtle -- the stated resolution from About this Mac matches the spec, but the displayable area appears to be far smaller. And Scaling is supposed to apply ONLY to Text, not to graphics, so your test document using .PNG should be at 1:1.
I think this may be a job for an Apple support -- and a display specialist at that. They make you work through a script with a First Responder. You can reference anything posted in this discussion (so no need to upload if already here) but their process is different and more rigorous, so they never take our conclusions at face value.
Be polite and professional, but if not solved, ask for a specialist. The specialist will likely have to call back, as there may be some work-from-home going on, and they are not likely to be standing around.