horizontal line does not extend all the way.

%\documentclass[11pt]{article}
\documentclass[letterpaper,11pt]{article}
\usepackage[left=0.9in, right=0.8in]{geometry}
%\usepackage{fullpage}
\usepackage{amsmath}
\usepackage[pdftex]{hyperref}
\hypersetup{
    colorlinks,%
    citecolor=black,%
    filecolor=black,%
    linkcolor=black,%
    urlcolor=black    % can put red here to visualize the links
}
\usepackage{amssymb}
\textheight=10in
\pagestyle{empty}
\raggedright

\def\bull{\vrule height 0.8ex width .7ex depth -.1ex }

% DEFINITIONS FOR RESUME
%-----------------------------------------------------------
\newcommand{\area}[2]{\vspace*{-9pt} \begin{verse}\textbf{#1}   #2 \end{verse}  }
\newcommand{\lineunder}{\vspace*{-8pt} \\ \hspace*{-18pt} \hrulefill \\}

\newcommand{\header}[1]{{\vspace*{3pt} \hspace*{-15pt}\vspace*{6pt} \textsc{#1}} \vspace*{-8pt} \lineunder}

\usepackage[usenames,dvipsnames]{color}
\usepackage{colortbl}
\definecolor{mygrey}{gray}{.90}

%\newcommand{\headergrey}[1]{\hspace*{-18pt} \colorbox{mygrey}
%   {\begin{minipage}
%   {\textwidth}{\textsc{#1}}
%   \end{minipage}}\lineunder}  % vphantom removed so greybox is thinner

%\newcommand{\headergrey}[1]{{\hspace*{-18pt} \colorbox{mygrey} {\begin{minipage}{\textwidth}{\textsc{#1 \vphantom{p\^{E}}}}\end{minipage}}\lineunder}}

%%-------------------------ANSWER 1------------------------------------------
\newcommand{\headergrey}[1]{%
    \noindent \hspace*{-15pt}%
    \begin{tabular}{p{\dimexpr\linewidth-3\tabcolsep+15pt}}
    \cellcolor{mygrey} \vspace{-7pt}\textsc{#1}\\
    \hline \end{tabular}%
    \par\nobreak \smallskip}%
%%-------------------------ANSWER 2------------------------------------------

%\newcommand{\headergrey}[1]{\hspace*{-18pt} \par
    %\noindent{\setlength\fboxsep{0pt}\colorbox{mygrey}
    %{\begin{minipage}{\dimexpr\textwidth-2\fboxsep\relax}
    %\vspace{2pt}%
    %\textsc{\strut#1}\par
    %\hrule
    %\end{minipage}}\par\smallskip}}
%%--------------------------------------------------------------------------------

\newcommand{\contact}[3]{ \vspace*{-55pt}
    \begin{center} {\LARGE \scshape {#1}}\\
    #2 \\ #3
    \end{center} \vspace*{-8pt} }

\newcommand{\school}[6]{
     \textbf{#1} #2 \hfill #3\\
      #4; #5 \hfill #6 \\ \vspace*{4pt} }

%-----------------------------------------------------------
% END RESUME DEFINITIONS

\begin{document}

\small
\smallskip
\vspace*{-44pt}

\contact{Name}
{Address}
{Number}

\headergrey{Objective}
    Seeking a Full Time position in the area of balh blah blah blah.
    \vspace*{6pt}

%\header{Education}
\headergrey{Education}
    \school {University of Latex}{Latex U.S.A.}{May 2013}{Masters of Science in Latex}{Adviser: }{GPA: xxxx}


\end{document}