为什么交点处的线不完全重合？

Question

为什么第二张图中的线不完全重合？

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{calc}
\usetikzlibrary{positioning,shapes.geometric,decorations.pathmorphing,decorations.pathreplacing,decorations.shapes,decorations.markings,patterns,calc,fit,arrows}
\usepackage{subfigure}

\begin{document}

    
\begin{figure}[ht]
    \centering
    \subfigure[Without acceleration of piston]{
    \begin{tikzpicture}[>=latex']
    \draw[thick,->] (0,0) -- (5,0) coordinate (x) node[below,midway]{Stroke length};
    \draw[thick,->] (0,0) -- coordinate[pos=0.5] (d1) (0,5) coordinate (y) node[left,midway]{Head};
    \draw[dashed] (d1) -- +($(x)-(0,0)$) coordinate (d2);
    \node[draw,rectangle, anchor=center, minimum height=3cm, minimum width=2cm] (r) at ($(d1)!0.5!(d2)$) {};
    \node [left] at (r.south west) {a};
    \node [right] at (r.south east) {b};
    \node [right] at (r.north east) {c};
    \node [left] at (r.north west) {d};
    \node [left] at ($(r.west)+(0,0.2)$) {e};
    \node [right] at ($(r.east)+(0,0.2)$) {f};
    
    \draw[->] ($(r.south west)+(0,-0.2)$) -- node[below] {suction} ($(r.south east)+(0,-0.2)$) ;
    \draw[->] ($(r.north east)+(0,0.2)$) -- node[above] {delivery} ($(r.north west)+(0,0.2)$) ;
    \draw[<->] ($(r.north west)+(0.5,0)$) -- node[right] {$H_d$} ($(r.west)+(0.5,0)$);
    \draw[<->] ($(r.east)+(-0.5,0)$) -- node[left] {$H_s$} ($(r.south east)+(-0.5,0)$);
    \draw [<-] ($(d1)+(0.3,0)$) -- +($(0,0)-(d1)$) node[right,near start]{$H_{atm}$};
    \end{tikzpicture}
    }\label{fig:indicator_wo_acceleration}
    \subfigure[With acceleration of piston]{
    \begin{tikzpicture}[>=latex']
    \draw[thick,->] (0,0) -- (5,0) coordinate (x) node[below,midway]{Stroke length};
    \draw[thick,->] (0,0) -- coordinate[pos=0.5] (d1) (0,5) coordinate (y) node[left,midway]{Head};
    \draw[dashed] (d1) -- +($(x)-(0,0)$) coordinate (d2);
    \node[draw,rectangle, anchor=center, minimum height=3cm, minimum width=2cm] (r) at ($(d1)!0.5!(d2)$) {};
    \node [left] at (r.south west) {a};
    \node [right] at (r.south east) {b};
    \node [right] at (r.north east) {c};
    \node [left] at (r.north west) {d};
    \node [left] at ($(r.west)+(0,0.2)$) {e};
    \node [right] at ($(r.east)+(0,0.2)$) {f};
    
    \draw (r.south west) -- ($(r.south west)+(0,-0.5)$) -- ($(r.south east)+(0,0.5)$);
    \draw[<->] ($(r.north west)+(0.5,0)$) -- node[right] {$H_d$} ($(r.west)+(0.5,0)$);
    \draw[<->] ($(r.east)+(-0.5,0)$) -- node[left] {$H_s$} ($(r.south east)+(-0.5,0)$);
    \draw [<-] ($(d1)+(0.3,0)$) -- +($(0,0)-(d1)$) node[right,near start]{$H_{atm}$};
    \end{tikzpicture}
    }\label{fig:indicator_w_acceleration}
    \caption{Indicator diagram of reciprocating pump}
    \label{fig:indicator-rp}
\end{figure}
\end{document}