Why is grouping used in the numerator of the definition of \frac but not the denominator?

Question

In the answers to this question: What is the difference between \over and \frac?, one finds the ams and plain latex definitions of \frac, respectively:

\DeclareRobustCommand{\frac}[2]{{\begingroup#1\endgroup\@@over#2}}
\DeclareRobustCommand\frac[2]{{\begingroup#1\endgroup\over#2}}

Is there a reason grouping is not used around #2? That is, like this:

\DeclareRobustCommand{\frac}[2]{{\begingroup#1\endgroup\@@over\begingroup#2\endgroup}}
\DeclareRobustCommand\frac[2]{{\begingroup#1\endgroup\over\begingroup#2\endgroup}}

Answer 1

declarations in the numerator of a primitive { ... \over ...} can leak in to the denominator, but an extra group in the denominator would do nothing.

\documentclass{article}
\begin{document}
[
{ a \over b}



{ \fam0 a \over b}



{\begingroup \fam0 a \endgroup \over b}
]
\end{document}