为什么 \hline 会产生不同粗细的线条

Question

我是 LaTeX 的新手，仍在学习中。我使用以下代码，惊讶地发现中间的水平线以粗体显示。任何帮助都将不胜感激。

 \begin{table}[htbp]
  \centering
  \caption{Average Returns for Three Years Following the Stock's Listing}
    \resizebox{\textwidth}{!}{\begin{tabular}{rrrccccc}
    \hline
          &       &       & First 6 months & Second 6 moths & First Year & Second Year & Third Year \\
    \hline
    IPO Firms &       &       & 144\% & 5.00\% & 152\% & 9.00\% & 5.00\% \\
    Size-matched Firms &       &       & 8\%   & 6.00\% & 17\%  & 16\%  & 7.00\% \\
    \textit{t}-statistics for difference &       &       & 2.03  & -0.41 & 1.8   & -1.09 & -0.21 \\
    Sample Size &       &       & 115   & 115   & 115   & 115   & 105 \\
    \hline
    \end{tabular}}
  \label{tab:returns}%
\end{table}%