The reason is that Book chapter has a character that extends below the baseline (the p) whereas the other two titles don't. Consider the following MWE, where the spacing is equal for the tree lines:
\documentclass{article}
\usepackage{currvita}
\usepackage{ragged2e}
\usepackage[margin=1in]{geometry}
\thispagestyle{empty}
\begin{document}
\section*{Edupation\hrule height 2pt}
abc
\section*{Peer-previewed Publications\hrule height 2pt}
abc
\section*{Book chapter\hrule height 2pt}
abc
\end{document}
Result:

There are other ways to get a line below each section title, for example with the titlesec package (from How can I make a bold horizontal rule under each section title?). Here the spacing is somewhat larger, which looks better than the original example for all three lines.
\documentclass{article}
\usepackage{currvita}
\usepackage{ragged2e}
\usepackage[margin=1in]{geometry}
\usepackage{titlesec}
\thispagestyle{empty}
\titleformat{\section}
{\normalfont\Large\bfseries}{\thesection}{1em}{}[{\titlerule[2pt]}]
\begin{document}
\section*{Education}
abc
\section*{Peer-reviewed Publications}
abc
\section*{Book chapter}
abc
\end{document}
Result:
