xcolor wrong conversion to cmyk color model

I get your cmyk-values with the following calculation steps:

  c= 1-r = 0.70195
  m= 1-g = 0.31374
  y= 1-b = 0.68626

  k = min(c,m,y) = 0.31374

  c = c-k = 0.38821
  m = m-k = 0
  y = y-k = 0.37252

This rather simple conversion method is described in the postscript manual (https://www.adobe.com/content/dam/acom/en/devnet/actionscript/articles/psrefman.pdf, page 305 ff). It is certainly one possibility to calculate the conversion.

But if you want to get completly confused about the "correct" conversion formula, you could read this thread https://forums.adobe.com/thread/428899.

Not really an answer. Just a code to visualize better the difference. As you can see below there is consistency across conversions. About the conversion in cmyk, as jfbu states in his/her comment, doesn't exist only one conversion algorithm because of device dependence.

\documentclass[border=10pt]{standalone}
\usepackage{xcolor}
\begin{document}
\sffamily
\begin{testcolors}[rgb,cmyk,hsb,HTML]
\testcolor[HTML]{4CAF50}
\testcolor[rgb]{0.29805,0.68626,0.31374}
\testcolor[cmyk]{0.38821,0,0.37251,0.31374}
\testcolor[cmyk]{0.5656,0,0.5429,0.3137}
\end{testcolors}
\end{document}