KOMA-Script like the standard classes use tabular environments to set the authors. The code is:
\begin{tabular}[t]{c}%
\@author
\end{tabular}
As you can see, the initial and the final column separator is not removed in this declaration. So if you misuse the font settings to try to force the author left aligned, only the tabular will be left aligned but the tabular still starts with a column separation space of width \tabcolsep.
You can use package xpatch to change the tabular declaration in \@maketitle. And if you already do so, you can also replace the center environment by a flushleft environment and avoid the ugly misusage of the font settings:
\documentclass[english]{scrartcl}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\setcounter{secnumdepth}{0}
\usepackage{xpatch}
\makeatletter
\xpatchcmd{\@maketitle}{\begin{center}}{\begin{flushleft}}{}{}
\xpatchcmd{\@maketitle}{\end{center}}{\end{flushleft}}{}{}
\xpatchcmd{\@maketitle}{\begin{tabular}[t]{c}}{\begin{tabular}[t]{@{}l@{}}}{}{}
\makeatother
\begin{document}
\title{My report}
\author{Joe User}
\maketitle
\section{Introduction}
Some intro
\end{document}
This patches can be used for the standard class article too.
If you have more than one author separated by \and, you may also change the definition of \and inside the definition of \maketitle, e.g.
\xpatchcmd{\maketitle}{%
\def\and{%
\end{tabular}%
\hskip 1em \@plus.17fil%
\begin{tabular}[t]{c}%
}%
}{%
\def\and{%
\end{tabular}\\
\begin{tabular}[t]{@{}l@{}}%
}%
}{}{}
to not longer have the authors side by side but one below the other. This last patch does not work for the standard classes. For the standard classes you can simply redefine \and:
\renewcommand*\and{%
\end{tabular}\\
\begin{tabular}[t]{@{}l@{}}%
}%
without \xpatchcmd.
If you want a clean solution: Do not use the default \maketitle if you don't like it. Define you own title if you need more freedom than the class provides.
xpatchis way much cleaner than fiddling withscrartcl.cls(-; ! Thanks! (... and +1 for last sentence!) – ebosi Apr 12 '17 at 14:23