Why my text in a fit node is not vertically centered?

Question

I have the following abstracted code. I do not know why the text test is not centered. Do you know how to make it centered?

\documentclass{standalone}

\usepackage{tikz}
\usetikzlibrary{positioning, math, fit, backgrounds}

\begin{document}

\begin{tikzpicture}
\node [draw = black] {A TEST};
\coordinate (a) at (current bounding box.south east); \draw (a) circle (3pt);
\coordinate (b) at ([yshift = -15 pt]current bounding box.south west);
\node (MEM) [inner sep = 0, fit = (a) (b), fill=black!20!red] {test};
\end{tikzpicture}

\end{document}

Result:

Please focus on the arrows I added manually.

Answer 1

This is the predefined behavior of TikZ, please see section 52 of the pgfmanual. You can place "test" in the center of the node by putting another node at the center of the MEM node.

\documentclass{standalone}
\usepackage{tikz}
\usetikzlibrary{positioning, math, fit, backgrounds}


\begin{document}

\begin{tikzpicture}
\node [draw = black] {A TEST};
\coordinate (a) at (current bounding box.south east); \draw (a) circle (3pt);
\coordinate (b) at ([yshift = -15 pt]current bounding box.south west);
\draw (b) circle (3pt);
\node (MEM) [inner sep = 0, fit = (a) (b), fill=black!20!red] {};
\node (test) at (MEM.center){test};
\end{tikzpicture}

Answer 2

In a "fited" node the text baseline is put at the center of the node. If you want your text to be vertically centered, you can use label=center:Test like this

\documentclass[tikz,border=7pt]{standalone}
\usetikzlibrary{fit}

\begin{document}
  \begin{tikzpicture}
    \node [draw = black] {A TEST};
    \coordinate (a) at (current bounding box.south east); \draw (a) circle (3pt);
    \coordinate (b) at ([yshift = -15 pt]current bounding box.south west);
    \node (MEM) [inner sep = 0, fit = (a) (b), fill=black!20!red, label = center:Test]{};
  \end{tikzpicture}
\end{document}