On my MacBook Pro, the System Report says that the resolution is "2560 x 1600 Retina". But when I use the [Free Ruler][1] app, it says that 1280 x 800.  That's with resolutions set to "Default for Display" in the system display settings.  

When the display setting is "Scaled" and "More Space", Free Ruler measures it as 1680 x 1050.

Why is there such discreptancy? What does "scaled resolution" even mean? How do I get the screen to operate at the resolution claimed.

To do my own research, I googled it.  But got seemingly irrelvant articles about points per inch and human eyes.  I asked ChatGPT.  Its answer was more relevant but less understandable.  I need human fingers to help me understand this, please.


  [1]: https://apps.apple.com/ie/app/free-ruler/id1483172210?mt=12