Unity’s model is equivalent to setting the CSS box-sizing property to border-box (mentioned in the doc here: https://docs.unity3d.com/Manual/UIE-USS-SupportedProperties.html).
See this page for details https://developer.mozilla.org/en-US/docs/Web/CSS/box-sizing.
The button in the repro project has a margin, that's why it has a small offset. A preferred way to center elements would be to use a combination of `align-items`, `justify-content` or `align-self`.