TikZ current page.north west shifted ~4pts?

TikZ current page.north west shifted ~4pts?

I'm trying to draw a background grid on a page, but current page.north
west is shifted down and to the right ~4pt. Anyone know how I can get the
exact north west corner of the page?
Here's an example:
\documentclass{article}
\setlength{\parindent}{0pt}
\usepackage[papersize={2.5in,0.5in},margin=0in]{geometry}
\usepackage{tikz}
\pagestyle{empty}
\begin{document}%
\begin{tikzpicture}[remember picture,overlay]%
\node at (current page.north west) [anchor=north west]{%
\tikz{\draw[step=0.25in,color=red,thick] (0,0)
grid (\paperwidth,\paperheight);}%
};%
\end{tikzpicture}%
\begin{minipage}{\linewidth}%
In Ti\emph{k}Z, \texttt{current page.north west} is shifted $\sim$4pt
right and down%
\end{minipage}%
\end{document}