Thanks for your answer Pilou.

There was a small mistake on my part.
The circle was not exactly round. Therefore, the center was not exactly in the middle of the circle.
I knew that with the helplines. ;)

Thanks anyway.

Yes the curves generator generate some troubles! It's a good habit to separate them!

You can also use the script of Max Smirnov for find the Center of Gravity! ;)
(just make some thickness to your surface! ;)

https://moi3d.com/download/scripts/max/CVolume.v.3.0.2022.05.24.zip