True surface area is >1

Maps are flat. The real world is sometimes flat, but quite often rugged or mountainous. That means the surface area of the real world is always some factor greater than the 2-D map projection. More mountainous regions will have a higher surface area than you'd think from just looking at a map.

That factor correction depends on the region under consideration. I looked at the mountainous area around Bolzano (Bozen) in the Alps. The correction factor can be as high as 1.6. The factor is 1.11x when averaged over a wide area around Bolzano that includes the various valleys and even some of the flatlands to the south.

Two observations. First, this factor is not accounted for when assessing land or assigning property tax. Second, we can increase the surface area of urban areas by large factors by increasing the roughness.