Problem with unexact font sizes in memoir

I have to typeset a theses and unless nobody knows why I have to set it in 12pt with one and a halve line spacing. The strange thing is, that in the resulting PDF file the font looks smaller and when I measure the font's size in the PDF file I get the result of: 11.96pt What could cause that?

Here the example:

\documentclass[a4paper,12pt,oneside]{memoir}

\usepackage{polyglossia} 
\usepackage{blindtext}


\newcommand*{\setasuspacing}[1]{%
\let\AsuSpacing#1
\AsuSpacing}
\setasuspacing{\OnehalfSpacing}

\begin{document}
\tableofcontents*
\chapter{Test, nothing else – Don't wast you time reading this! It has to have two lines}
\section{Don't wast you time reading this! It has to have two lines. It has to have two lines}
\Blindtext
\footnote{\blindtext}
\blindtext
\section{Test124}
\Blindtext
\chapter{Test2}
\Blindtext \Blindtext \Blindtext

\end{document}