Why the second caption is not centered?

Question

I'm using the following code to show two tikzpictures in a line.

\documentclass[11pt]{article}
\usepackage{tikz}
\usepackage[labelformat=empty]{caption}
\begin{document}
\begin{center}
    \begin{minipage}[b]{.5\textwidth}
        \centering
        \begin{tikzpicture}
        \draw (0,2) -- (0,-2);
        \end{tikzpicture}
        \captionof{figure}{
            A non-essential apartment\\ 
            of type $A_1$.}
        \end{minipage}%
        \begin{minipage}[b]{.5\textwidth}
        \centering
        \begin{tikzpicture}
            \foreach \x in {0,0.6,1.2,1.8}
                \draw (\x,2)--(\x,-2) (2,\x)--(-2,\x) (-\x,2)--(-\x,-2) (2,-\x)--(-2,-\x);
        \end{tikzpicture}
        \captionof{figure}{
            A non-irreducible apartment\\ 
            of type $A_1\times A_1$.}
        \end{minipage}
\end{center}
\end{document}

However, it shows as following

Why the second captionof is not centered?

Update: shorten the text in second captionof to show the problem more clear.

Answer 1

the first caption, without the \\ would fit on a single line, this causes a different route to be taken, but then finally the \\ is executed in the scope of centering and the lines are centred.

The second caption does not fit on a line so is set in a parbox.

Here I used the caption package setup command to skip the check for single line, so both captions are treated the same way. If you want the captions to be centred in their parbox (or have just the last line of a justified caption centred, the caption package has options for that.

\documentclass[11pt]{article}
\usepackage{tikz}
\usepackage[labelformat=empty]{caption}
\begin{document}
\begin{center}
\captionsetup{singlelinecheck=off,justification=centerlast}
\begin{minipage}[b]{.5\textwidth}
        \centering
        \begin{tikzpicture}
        \draw (0,2) -- (0,-2);
        \end{tikzpicture}
        \captionof{figure}{
            A non-essential apartment\\
            of type $A_1$.}
        \end{minipage}%
        \begin{minipage}[b]{.5\textwidth}
        \centering
        \begin{tikzpicture}
            \foreach \x in {0,0.6,1.2,1.8}
                \draw (\x,2)--(\x,-2) (2,\x)--(-2,\x) (-\x,2)--(-\x,-2) (2,-\x)--(-2,-\x);
        \end{tikzpicture}
        \captionof{figure}{
            A non-irreducible essential apartment
            of type $A_1\times A_1$.}
        \end{minipage}
\end{center}
\end{document}