>Why this corner is so distorted ?

I am not sure what do you mean with distorted. It is a poly-surface (filleted cube G1) and then intersected lines.

>Does exist a rational reason of that ?
I guess the reason why 'this' exist it is cause I exist. Which is the same reason why things (not talking about nature)
have a shape, someone existed. This is not a challenging answer, I actually wonderer why this shape exist.

>When a form is not "natural" it's normal that you obtain such difficulties! :)

Can you clarify what should be 'natural' . Again you are challenging my perspectives (which is good). There are thousands cars
I can see from my window now. Do you believe they are 'natural' ? Or they are more logical for a cad software than a cube with radius ?

I apologize if this sounds too much. The truth I am frustrated. I am wondering how is possible I saw so many crazy shapes on CAD software.
I ve seen an AUDI R8 in CAD, Its been weeks making a cube with G1 filleted edges and grooves :(

Thanks a lot for participating. I am looking forward for suggestions :)