UI Constant Physical Size, rect sizes not corresponding to real sizes

Hello community! After reviewing various topics about "constant physical size" of UI elements, it seemed that most of the problems were "Editor visualization".

For particular needs, I want to have an image of exactly 20mm x 20mm, on every device. I tought that, setting Canvas Scaler to constant physical size, setting millimeters, and setting "width / heigth" of the image to 20, would do the job.

Unfortunately, it doesn't. Measuring the image on actual screen devices with a ruler, shows that the image is a little bigger, with small differences with different devices. (In editor: 2cm x 2cm, Real screen: 2.4cm x 2.4cm)

Canvas / Canvas scaler configuration

Image configuration

Am I doing something wrong? What do I need to do to achieve exactly the size I want?

Thank you in advance for your kindness!