U factor does not change with the size

[Cinzia]

Hi, 

I created a window using the custom single vision type of size 2000x2000 mm. I have not modified the materials, I have used those already present in the window. The U factor is 5.813.

I then created a copy of the same window by changing the dimensions 1200x2000. The U factor is 5.808.

How is it possible that the U factor of the whole window does not change?

I used the CEN mode.

Thanks for your help