Wrong positioning of subscripts on bold math letters

Question

I just noticed that for kpfonts and times (and probably other fonts since it does not seem to be font-related), the subscripts on boldface letters like V are (I would say) wrong. Is there an explanation?

\documentclass{book}
\usepackage{times}
%\usepackage{kpfonts}
\begin{document}
$\mathbf{A}_a$ $A_a$ $\mathbf{V}_a$ $V_a$
\end{document}

Answer 1

Implicit kerning is not taken into account for math letters and their subscripts. Only the italic correction is considered. As workaround the spacing can be manually fixed, e.g.:

\documentclass[a4paper]{book}
\pagestyle{empty}
\begin{document}
$\mathbf{A}_a^a$ $A_a^a$ $\mathbf{V}_a^a$ $V_a^a$
$\mathbf{V}_{\!\!a}$ $V_{\!a}$
\end{document}

Package times does not matter, because it was only used for the page number.