Dimensions of spheres are wrong I have created three spheres (that together should make up the earth). I first created the inner sphere (the earth surface). Then I duplicated that and set the x,y,z scales at 1.012 (this sphere is for the clouds). 

After that I created the third sphere (atmosphere) and set the scales at 1.017. No - a few weeks later - I have a look at the file and see that the x,y,z dimensions of these spheres are wrong. I expect the x,y,z dimensions to be the same for one sphere. How is this possible ? 

By the way ... when I created these spheres I was doing this Blenderguru tutorial and as far as I know his dimensions were "perfect" for all three spheres.

The earth


Cloud sphere

Atmosphere


I am not that familiar with Apply Scale ... but should  I have done that ? What could have happened ?

And is there a way to get the dimensions and scaling right (next to create the spheres from scratch) ?
This is because it is not a perfect sphere, it is constructed of flat faces and it is not perfectly symmetrical. ( could have a vertex at the top and a flat face at the bottom), to get it  close to the dimensions you want increase the resolution of the sphere (segments and rings ).

just consider a simple case of a circle since it is made of finite straight lines its dimension wouldn't be as expected depending on its resolution :



you can clearly see why X differ from Y for this circle.