why does rotation style work on actual coordinates and not variables in tikz 3d plot?

Question

 \documentclass{minimal}
\usepackage{tikz,tikz-3dplot}

\begin{document}

  \tdplotsetmaincoords{70}{115}

  \begin{tikzpicture}[scale=4,tdplot_main_coords]
\pgfmathsetmacro{\rvec}{.8}
\pgfmathsetmacro{\thetavec}{30}
\pgfmathsetmacro{\phivec}{60}

\tdplotsetrotatedcoords{\phivec}{\thetavec}{0}

 \coordinate (O) at (1,1,1);

 %these 2 lines overlap
\draw[thick,->] (O) -- (0,.5,0);
\draw[thick,tdplot_rotated_coords,->] (O) -- (0,.5,0);

 %these 2 lines dont overlap
\draw[thick,->] (1,1,1) -- (0,.5,0);
\draw[thick,tdplot_rotated_coords,->] (1,1,1) -- (0,.5,0);
\end{tikzpicture}
\end{document}