Why is the Sun less massive than other G2V stars? The Wikipedia page for G-type main-sequence stars lists the expected mass and temperature for stars of each sub-classification:
$$\begin{array}{c|c|c|} 
 &amp; \text{Mass ($M_\odot$) } &amp; \text{Temperature (K)} \\ \hline
\text{G0V} &amp; 1.15 &amp; 5980 \\ \hline
\text{G1V} &amp; 1.10 &amp; 5900 \\ \hline
\text{G2V} &amp; 1.07 &amp; 5800 \\ \hline
\text{G3V} &amp; 1.04 &amp; 5710 \\ \hline
\text{G4V} &amp; 1.00 &amp; 5690 \\ \hline
\text{G5V} &amp; 0.98 &amp; 5620 \\ \hline
\text{G6V} &amp; 0.93 &amp; 5570 \\ \hline
\text{G7V} &amp; 0.90 &amp; 5500 \\ \hline
\text{G8V} &amp; 0.87 &amp; 5450 \\ \hline
\text{G9V} &amp; 0.84 &amp; 5370 \\ \hline
\end{array}$$
Spectral type is only determined through a star's temperature, and thus the Sun with a temperature of $\text{5778 K}$ is unambiguously a G2V star.
However the Sun is noticeably less massive than other G2V stars are. Why is that? Or, another way of asking, why is the Sun hotter than other stars of the same mass (G4V)?
It isn't. You've just got  dodgy table from wikipedia.
A more modern (and well-used) version is here.
It lists
G1V  1.07 5880
G2V  1.02 5770
G3V  1.00 5720
This is an average relationship. The closest and most consistent relationship will be between spectral type and effective temperature and indeed the Sun is normally attributed a spectral class of G2V and $T_{\rm eff} \simeq 5780$ K.
The reason for any small discrepancy at the hundredth of a solar mass level with the mass in the table could be because the Sun has a slightly different composition to the average star that defines this table. The age of the Sun is unlikely to be a major factor. According to most models, the temperature of the Sun increases by just 100 K between a billion years old and around 8 billion years old and then cools a little bit. On the other hand, stars like the Sun do lose a little mass as they get older, mainly through a stellar wind. The Sun though is probably at around the average age for an early G star in our Galactic neighbourhood.