Struggling with the border size for a constant border

According to darktable 3.8.1 and manual 3.8 the border size of the frame is a percentage of the underlying full image.
Thus for the size of an underlying full square image of 1000 x 1000 px and a constant border size of 10 percent I would expect a total size of 1100 x 1100 px (i.e. full underlying image size plus frame).
However, the actual size after exporting is 1111 x 1111 px.
And for an underlying full image of 3000 x 2000 px and a constant border size of 10 percent the resulting image plus frame becomes 3333 x 2333 px.
Perhaps I misunderstand the specification in the manual, but for me these numbers are not the size of the underlying full image plus 10 percent of it for a constant border size. Could someone clarify this to me, please?

You are right, the manual seems to be wrong here. The tooltip looks more accurate:


So it’s 10% of the final size of the edited image (or longer side of it when set to “constant border”). So, 10% of 3333 px.

Thus, “final size” means size of the original image plus size of the frame.
Thanks a lot for this clarification!