とあるマクロの挙動がおかしいので調べてみると，
以下のようになりました．
\usepackage{emath}をコメントアウトすればいずれも
正しく 7.5, 2.5, 7.5 となります．
これはそういうものなのでしょうか．

\documentclass[fleqn]{jarticle}
\usepackage{emath}
\begin{document}
\makeatletter
\settoheight{\dimen@}{$\mathstrut$}%
\settodepth{\dimen@i}{$\mathstrut$}%
\settoheight{\dimen@ii}{$\mathstrut$}%
$D_0=\the\dimen@,~D_1=\the\dimen@i,~D_2=\the\dimen@ii.$
% OK 7.5, 2.5, 7.5

\settoheight{\dimen@}{$\mathstrut$}%
\settoheight{\dimen@ii}{$\mathstrut$}%
\settodepth{\dimen@i}{$\mathstrut$}%
$D_0=\the\dimen@,~D_1=\the\dimen@i,~D_2=\the\dimen@ii.$
% NG 7.5, 2.5, 2.5

\settodepth{\dimen@i}{$\mathstrut$}%
\settoheight{\dimen@}{$\mathstrut$}%
\settoheight{\dimen@ii}{$\mathstrut$}%
$D_0=\the\dimen@,~D_1=\the\dimen@i,~D_2=\the\dimen@ii.$
% OK 7.5, 2.5, 7.5

\settodepth{\dimen@i}{$\mathstrut$}%
\settoheight{\dimen@ii}{$\mathstrut$}%
\settoheight{\dimen@}{$\mathstrut$}%
$D_0=\the\dimen@,~D_1=\the\dimen@i,~D_2=\the\dimen@ii.$
% OK 7.5, 2.5, 7.5

\settoheight{\dimen@ii}{$\mathstrut$}%
\settoheight{\dimen@}{$\mathstrut$}%
\settodepth{\dimen@i}{$\mathstrut$}%
$D_0=\the\dimen@,~D_1=\the\dimen@i,~D_2=\the\dimen@ii.$
% NG 7.5, 2.5, 2.5

\settoheight{\dimen@ii}{$\mathstrut$}%
\settodepth{\dimen@i}{$\mathstrut$}%
\settoheight{\dimen@}{$\mathstrut$}%
$D_0=\the\dimen@,~D_1=\the\dimen@i,~D_2=\the\dimen@ii.$
% OK 7.5, 2.5, 7.5

\makeatother
\end{document}

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└◆6660:\dimen@??がうまく使えない [tommy_minor] 11/08 14:40
　└◆6661:Re:\dimen@??がうまく使えない [田中徹] 11/08 15:10
　　└◆6662:Re[2]:\dimen@??がうまく使えない [tommy_minor] 11/08 15:59
　　　└◆6663:Re[3]:\dimen@??がうまく使えない [tDB] 11/08 16:33
　　　　└◆6666:Re[4]:\dimen@??がうまく使えない [tommy_minor] 11/08 21:42
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