[texhax] Plain TeX: text body not centered?

[Rodolfo Medina]

In plain TeX, the simple file:


 \hoffset=0pt
 \centerline{ollo}
 \vskip 1cm
 \hrule

 \bye


produces an output that's not centered in the middle of the page although
\hoffset is set to 0.  Why?

Thanks for any help
Rodolfo