You have an error in the documentation

[Airon65]

https://www.imagemagick.org/Magick++/Drawable.html

DrawableRoundRectangle

double centerX_, double centerY_, double width_, double hight_, double cornerWidth_, double cornerHeight_

centerX, centerY - it's not the center. It's just the leftTop position.
hight_ - misspelling (height).
width, height - it's not the width and height. It's rightBottom position of the figure.

IM 7, macOS 10.12.3

[dlemstra]

Thanks for this correction. The next version of ImageMagick will use the correct names in the constructor. There will be new property names for upperLeft and lowerRight but we will also keep the old ones for backwards compatibility.