How to understand behavior of beamercolorbox with regard to width?

The manual of beamer 3.59 on page 125 reads as follows:

If the width is larger than the normal text width, as specified by the
value of textwidth, the width of the resulting box is reset to the
width textwidth, but intelligent negative skips are inserted at the
left and right end of the box.

I am trying to match this with the result of the following minimal example:

documentclass[aspectratio=169]{beamer}
setbeamercolor{bgcolor}{fg=black,bg=red}
setbeamertemplate{frametitle}{%
begin{beamercolorbox}[wd=dimexprpaperwidth+1.0cmrelax]{bgcolor}Boxborderhfill redrobxoBend{beamercolorbox}}
begin{document}
begin{frame}{Title}
Paperwidth: thepaperwidth

Textwidth: thetextwidth
end{frame}
end{document}

My problem:

The wd of the colorbox is set to paperwidth+1.0cm and clearly larger than textwidth. However the width of the box does not get reset to the width of textwidth but fills even more than the paperwidth. It is reported as overfull and 28.45274pt too wide. But even if we deduct this overfull dimension from the paperwidth (as we can see visually, the box is at least as wide as the paperwidth), one still does not come anything near to the textwidth, which is a whopping 57pt smaller than the paperwidth.

Is there a bug in the documentation or in the implementation or in my understanding? And if the latter: What is the nature of the misunderstanding?