Standalone graphic has still borders left and right

Question

I am working with small graphics which must not have any border. Surprisingly for me this code produces small (1-2mm) borders left and right. Could you enlighten me?

This code...

\documentclass[tikz=true,border=0pt]{standalone}
\begin{document}%
\begin{tikzpicture}[x=1mm, y=1mm]%
    \draw [line width=5mm, color=green] (0,0) -- +(5,0) {};
    \draw [line width=5mm, color=red] (5,0) -- +(5,0) {};
    \draw [line width=5mm, color=green] (10,0) -- +(5,0) {};
    \draw [line width=5mm, color=red] (15,0) -- +(5,0) {};
\end{tikzpicture}%
\end{document}

..produces this output:

Answer 1

As asked by the OP, here is a fix to get rid of the border lines that however does not tell why they were there in the first place:

\documentclass[tikz=true,border=0pt]{standalone}
\begin{document}%
\begin{tikzpicture}[x=1mm, y=1mm]%
    \draw [fill=green, color=green] (0,0) rectangle +(5,5);
    \draw [fill=red, color=red] (5,0) rectangle +(5,5);
    \draw [fill=green, color=green] (10,0) rectangle +(5,5);
    \draw [fill=red, color=red] (15,0) rectangle +(5,5);
\end{tikzpicture}%
\end{document}

Answer 2

The lines normally are extended a bit to take into account the "caps" (see why in
Schrödinger's cat's answer here).

You can see what's happening if you use:

\begin{tikzpicture}[x=1mm, y=1mm, line cap=round]%

and you'll obtain:

The manual explains this:

You can use rect, but the output is funny:

add transparency to see what's happening):

\documentclass[tikz=true,border=0pt]{standalone}
\begin{document}%
\begin{tikzpicture}[x=1mm, y=1mm, line cap=rect]%
    \draw [line width=5mm, color=green, opacity=0.4] (0,0) -- +(5,0) {};
    \draw [line width=5mm, color=red, opacity=0.4] (5,0) -- +(5,0) {};
    \draw [line width=5mm, color=blue, opacity=0.4] (10,0) -- +(5,0) {};
    \draw [line width=5mm, color=yellow, opacity=0.4] (15,0) -- +(5,0) {};
\end{tikzpicture}%
\end{document}