pgfmathsetmacro increases border size

Question

Upon creating some graphs with TikZ, I noticed that the border at one side was much larger than it needed to be. The cause seems to be the \pgfmathsetmacro command:

\documentclass{standalone}
\usepackage{tikz}
\begin{document}

%\pgfmathsetmacro{\a}{0}
%\pgfmathsetmacro{\b}{0}
%\pgfmathsetmacro{\c}{0}
%\pgfmathsetmacro{\d}{0}
%\pgfmathsetmacro{\e}{0}

\begin{tikzpicture}
\draw (0, 0) rectangle (1, 1);
\end{tikzpicture}

\end{document}

Without any defined macros, the output looks like this (Adobe reader background is gray):

With each macro, the size increases. With all five macros defined, it looks like this:

How can this behaviour be explained? Am I using the \pgfmathsetmacro in a wrong way? I did this a lot in the past and never had such a problem. The following did not help, the border didn't change at all:

\documentclass[border=0pt]{standalone}