documentclass{article}
usepackage{fancyhdr,graphicx,tcolorbox}
begin{document}
pagestyle{fancy}
noindent
parindent=0pt
makebox[0pt][l]{some text}% typeset-1
rotatebox[origin=lb]{90}{some text}\[3em]% typeset-2

rotatebox{0}{% typeset-3
  begin{tcolorbox}[width=0.7textwidth,height=6em]
    the left side of this box alligns well with the left end of the headrule.
  end{tcolorbox}
}\
%
rotatebox[origin=lb]{90}{% typeset-4
  begin{tcolorbox}[width=0.7textwidth,height=6em]
    As the option "origin=lb" is used, the right side of this rotated box should align with the left end of the headrule, but it does not.
  end{tcolorbox}
}
end{document}