Unit of panorama skybox

I set panorama image into skybox and I place object onto panorama image.

I found that the scale is 1m in real world is 10 unit in Unity3D at the skybox.

At first I think the 10 value would depend on panorama image (or camera focal length). So I checked other images.

However, the scale value is surprisingly same.

I think all scale is same as 10 in any panorama image.

I want to know the background this for having conviction.

There is no “scale” for a panoramic image. Try scaling your whole scene around the camera position and you will see that the image the camera sees does not change at all.