为什么我的表格标题移线！

Question

\documentclass[conference]{IEEEtran} 
\usepackage{graphicx} 
\usepackage{amsmath} 
\usepackage[english]{babel} 
\usepackage[utf8x]{inputenc} 
\hyphenation{op-tical net-works semi-conduc-tor IEEEtran} 
\begin{document}
\begin{table}[h!]
\caption{Arnold period in different size images}
\setlength{\tabcolsep}{8pt}
\renewcommand{\arraystretch}{1.5}
\centering
\begin{tabular}{|c|c|c|c|c|c|c|}
    \hline
    Size N & 4*4 & 8*8 & 32*32 & 64*64 & 128*128 & 256*256\\
    \hline
    Period M & 3 & 6 & 24 & 48 & 96 & 192\\
    \hline
\end{tabular}
\end{table}
\end{document}