You wrote,
Here, the math expressions a
and b
are placed in an italic context. However, it seems that they receive more spacing on the right than the left. Is this observation true?
The answer depends very much on the letter that's rendered either in math mode or in text-italic mode in the italics group. Here are several examples -- the black part uses math mode, and the superimposed red part uses text-italic mode:

For the a-b example you considered in your posting, it is true that the combined whitespace on either side of a
and b
is about the same for both a
and b
, and also that there's more asymmetry -- less whitespace to the left and more whitespace to the right -- for the math mode case than for the text-italic case.
More generally, though, the exact differences in the horizontal offsets depend both on which letters are rendered in math mode and on the letter(s) -- if any -- that immediately precede the space before the math mode material.
\documentclass[border=1pt,preview]{standalone}
\usepackage{xcolor}
\newlength\mylen
\newcommand\blurba[2]{\textit{that $#1$ and $#2$ are}}
\newcommand\blurbb[2]{\textit{that #1 and #2 are}}
\newcommand\mykern[2]{\settowidth{\mylen}{\blurba{#1}{#2}}%
\kern-\mylen}
\newcommand\blurb[2]{\blurba{#1}{#2}%
\mykern{#1}{#2}%
\textcolor{red}{%
\blurbb{#1}{#2}}}
\begin{document}
\obeylines
\blurb{a}{b}
\blurb{d}{l}
\blurb{u}{v}
\blurb{x}{z}
\end{document}