2700 x 1800 is the native screen resolution of the original 8.1″ Surface Duo when unfolded. Like the rest of the family, it has a 3:2 aspect ratio display. It has a total of 4.86 million pixels with a high 401 PPI pixel density.
1800 x 1350 is the native screen resolution when the device is folded. The aspect ratio now becomes 4:3 with the same 401 PPI pixel density.
You can learn more about screen information for all Surface devices from the Surface display comparison table we made here.