Summation symbol different for two similar cases

Question

Why in the above formula the subscript and superscript for summation symbol is different from the below one?

\documentclass[hidelinks,review]{elsarticle}
\usepackage{amsmath}
\usepackage{mathtools}
\begin{document}
\begin{equation}
\textrm{R}^{2}=1-\frac{\sum_{i=1}^{n}(a_{i}-p_{i})^{2}}{(\mu_{a}-p_{i})^{2}}
\end{equation}
\begin{equation}
\textrm{RMSE}=\sqrt{\frac{1}{n}\sum_{i=1}^{n}(a_{i}-p_{i})^{2}}
\end{equation}
\end{document}

please note that it is numbered formula

Answer 1

As explain @daleif in his comment, nominators and denominators in fractions are set in text-mode math. This can be changed by \displaystyle and \limits:

\documentclass[hidelinks,review]{elsarticle}
\usepackage{mathtools}
\begin{document}
\begin{equation}
\mathrm{R}^{2}=1-\frac{\sum_{i=1}^{n}(a_{i}-p_{i})^{2}}{(\mu_{a}-p_{i})^{2}}
\end{equation}
\begin{equation}
\mathrm{R}^{2}=1-\frac{\sum\limits_{i=1}^{n}(a_{i}-p_{i})^{2}}{(\mu_{a}-p_{i})^{2}}
\end{equation}
\begin{equation}
\mathrm{R}^{2}=1-\frac{\displaystyle\sum_{i=1}^{n}(a_{i}-p_{i})^{2}}{(\mu_{a}-p_{i})^{2}}
\end{equation}
\begin{equation}
\mathrm{RMSE}=\sqrt{\frac{1}{n}\sum_{i=1}^{n}(a_{i}-p_{i})^{2}}
\end{equation}
\end{document}