Slight text lowering

Please note the following simple example typed in Plain TeX code (that I
process with simply: `tex file'):

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\voffset=-1true in

\font\sixteenrm=cmr16\sixteenrm

Hello!

\vfill\eject\end
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

and look how the text perfectly touches the top of the paper.  Instead, if I
comment out the line including the font statement, the text slightly lowers
down a bit.  Is it normal?

Thanks,

Rodolfo