垂直轴上的小网格间距似乎不一致

Question

有人能向我解释一下为什么垂直轴上 0 到 200 之间的间距似乎在 200 和 400 之间不同吗？

\documentclass[tikz, border=5mm]{standalone}
\usepackage{pgfplots}
\pgfplotsset{compat=1.17}

\begin{document}

\begin{tikzpicture}
    \begin{axis}[
        xlabel={Time (years)},
        ylabel={Activity (Bq)},
        domain=0:2000,
        samples=100,
        axis x line=bottom,
        axis y line=left,
        xtick={0,200,...,2000},
        ytick={0,200,...,2000},
        ticklabel style={font=\footnotesize},
        x tick label style={rotate=45, anchor=east},
        black,
        width=10cm,
        height=10cm,
        grid=both,
        minor y tick num=4,
        minor x tick num=4,
        major grid style={black, line width=1pt},
        minor grid style={black!30, line width=0.8pt},
        ]
        \def\Nzero{2000}
        \def\halfLife{432}
        \def\decayConstant{ln(2)/\halfLife}
        
        \addplot [thick] {\Nzero * exp(-\decayConstant * x)};
    \end{axis}
\end{tikzpicture}

\end{document}