Inspired by this, here is an MWE that show a visible margin between the end of the row and the right hand side of the table. That's annoying. Any idea how to remove that?
\documentclass[12pt]{article}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% Fancy table
% https://tex.stackexchange.com/questions/112343/beautiful-table-samples
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\usepackage[usenames,dvipsnames]{xcolor}
\usepackage{tcolorbox}
\usepackage{tabularx}
\usepackage{array}
\usepackage{colortbl}
\tcbuselibrary{skins}
\usepackage{multirow}
\newcolumntype{Y}{>{\raggedleft\arraybackslash}X}
\newcolumntype{Z}{>{\centering\arraybackslash\hsize=.6\hsize}X}
\tcbset{tab2/.style={enhanced,fonttitle=\bfseries,fontupper=\normalsize\sffamily,
colback=yellow!10!white,colframe=red!50!black,colbacktitle=Salmon!40!white,
coltitle=black,center title}}
\begin{document}
\begin{tcolorbox}[tab2,tabularx={Z || Z | Z | Z},title=AAA,width=0.7\textwidth]
BBB & BBB & BBB & BBB \\ \hline\hline
\multirow{3}{*}{XXX} & AAA & AAA & AAA \\ \cline{2-4}
& & AAA & AAA \\ \cline{2-4}
& & AAA & AAA
\end{tcolorbox}
\end{document}


\newcolumntype{Z}{>{\centering\arraybackslash}X}– Salim Bou Nov 30 '16 at 12:52