两个垂直对齐的小页面水平移动

Question

我想使用 minipages 将两个列表对齐。不幸的是，右侧 minipage 稍微向底部移动了一点。这是什么问题？

\documentclass[11pt,fleqn,a4paper,pdftex,twoside]{book}
\usepackage[utf8]{inputenc}
\usepackage[english]{babel}
\usepackage{parcolumns}
\usepackage{listings}

\lstdefinestyle{customc}{
    language=C,
    tabsize=2,
    breaklines=true,
    captionpos=b,
    frame=tlrb
}
\begin{document}

\noindent\begin{minipage}{.48\textwidth}
    \begin{lstlisting}[style=customc, caption={Unoptimized if statement in GetFilteredNewHostCommand}]
if (tmp != NULL)
    \end{lstlisting}
\end{minipage}\hfill
\begin{minipage}{.48\textwidth}
    \begin{lstlisting}[style=customc, caption={Optimized if statement in GetFilteredNewHostCommand}]
if (unlikely(tmp != NULL))
    \end{lstlisting}
\end{minipage}
\end{document}