Is that extra empty space below the `\xRightarrow`?

As I was browsing this excellent solution, it seemed that there exist extra empty space below the arrow:

Is this a small bug or feature designed intentionally? Or is that elegant method to balance these two space's height?

Here below is my test code:

\documentclass{article}\usepackage{mathtools}\begin{document}case1: $\xRightarrow[aeiou]{aeiou}$case2: $\xRightarrow[aeiou]{pqgqp}$case3: $\xRightarrow[hbdlf]{aeiou}$case4: $\xRightarrow[hbdlf]{pqgqp}$case5: $\xRightarrow[ABCDE]{FGHIJ}$\end{document}

Edited according to egreg's answer:

As egreg figured out that this issues is "font-specific", with fourier, the \xRightarrow would be vertically centered better(but still not exactly...) However, the = is still not centered.

Now I still wonder what is the best practice to tune the "below"part's height(mainly focus on the default Computer Modern Math/ with unicode-math's Latin Modern Math)?

\documentclass{article}\usepackage{mathtools}\usepackage{graphicx}\newcommand*\myxRightarrow[2][]{%    \xRightarrow[\raisebox{4pt}{\scalebox{.65}{{$#1$}}}]{#2}}%\begin{document}case1: $\xRightarrow[aeiou]{aeiou}$ \quad $\myxRightarrow[aeiou]{aeiou}$case2: $\xRightarrow[aeiou]{pqgqp}$case3: $\xRightarrow[hbdlf]{aeiou}$case4: $\xRightarrow[hbdlf]{pqgqp}$case5: $\xRightarrow[ABCDE]{FGHIJ}$\end{document}