# Is there any other margin I must set to reduce margin to zero?

I have basic class file and I would like to set margins of an a4paper to 0mm. Here is the markup of some-class.cls:

\NeedsTeXFormat{LaTeX2e}
\ProvidesClass{some-class}[2013/07/29]

\renewcommand{\normalsize}{\fontsize{10pt}{12pt}\selectfont}
\DeclareOption{a4paper}{
\setlength\paperheight {297mm}%
\setlength\paperwidth  {210mm}
}
\DeclareOption{landscape} {
\setlength\@tempdima   {\paperheight}%
\setlength\paperheight {\paperwidth}%
\setlength\paperwidth  {\@tempdima}
}

%Default options for design
\ExecuteOptions{a4paper}
%Process user given options
\ProcessOptions

\RequirePackage[top=0mm, bottom=0mm, left=0mm, right=0mm, textwidth=210mm]{geometry}

Content of my document.tex:

\documentclass[]{some-class}
\author{roncsak}
\title{some title}
\date{2013/07/28}
\begin{document}
Lorem ipsum
\end{document}

I would like to set an A4 paper with zero margins. Instead I notice some margin on the left side (approximately 10mm). I could reduce that margin with the \RequirePackage[top=0mm, bottom=0mm, left=-10mm, right=0mm, textwidth=210mm]{geometry} but I would like to understand why this margin left behind.

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