Understanding the subscriptShiftDown and subscriptShiftUp OpenType Math parameters

As per Microsoft's Typography documentation, the subscriptShiftDown parameter has the description

The standard shift down applied to subscript elements. Positive for moving in the downward direction. Suggested: os2.ySubscriptYOffset.

and the superscriptShiftUp parameter has the description

Standard shift up applied to superscript elements. Suggested: os2.ySuperscriptYOffset.

So, when I use the \Umathsubshiftdown and \Umathsubshiftup commands to change these parameters to zero, I get that the baselines of the subscripts, superscripts and the main glyphs are not aligned. Why?

\documentclass{article}

\usepackage{ifluatex}
\usepackage[newcmbb]{fontsetup}

\ifluatex
  \everymath=\expandafter{%
    \the\everymath%
    \Umathsubshiftdown\textstyle=0pt}
  \everydisplay=\expandafter{%
    \the\everydisplay%
    \Umathsubshiftdown\displaystyle=0pt}
  \everymath=\expandafter{%
    \the\everymath%
    \Umathsupshiftup\textstyle=0pt}
  \everydisplay=\expandafter{%
    \the\everydisplay%
    \Umathsupshiftup\displaystyle=0pt}
\fi

\begin{document}
    \centering \(a_b a^b \symbb{R}^2 \symup{C}^\infty\)
\end{document}