  The STAR isobar data of $^{96}$Ru+$^{96}$Ru and $^{96}$Zr+$^{96}$Zr
collisions at $\sqrt{s_{\mathrm{NN}}}= 200$ GeV show that ratios of observables
($R_{\mathcal{O}}$) such as the multiplicity distribution,
$p(N_{\mathrm{ch}})$, and the harmonic flow, $v_n$, deviate from unity, when
presented as a function of centrality, $c$. These deviations have been
attributed to the differences in the shape and radial profiles between
$^{96}$Ru and $^{96}$Zr nuclei. In addition, the ratios $R_{\mathcal{O}}(x)$
depend on the choice of the event activity variable $x$, which could be either
$N_{\mathrm{ch}}$ or centrality. We estimate the difference $\Delta R$ between
these two choices, based on the published $p(N_{\mathrm{ch}})$, as well as
those from a multiphase transport (AMPT) model with varied nuclear structure
parameters: nuclear radius ($R_0$), surface diffuseness ($a_0$), quadrupole
deformation ($\beta_2$), and octupole deformation ($\beta_3$). In contrary to
$R_{v_n}(c)$, $R_{v_n}(N_{\mathrm{ch}})$ is nearly independent of the analysis
approaches, suggesting that nonflow effects are better controlled by
$N_{\mathrm{ch}}$ than $c$. The ratios of observables sensitive to the chiral
magnetic effect (CME) are also much closer to unity for $x=N_{\mathrm{ch}}$
than $x=c$, indicating that the ratios calculated at the same $N_{\mathrm{ch}}$
provide a better baseline for the non-CME background. According to the AMPT
results, the dominant parameter for $\Delta R$ is $a_0$, while $R_0$ and
$\beta_n$ are only important in central collisions. The published
$p(N_{\mathrm{ch}})$ is also used to estimate $\Delta R_{\left\langle
p_{\mathrm{T}}\right\rangle}$ for mean transverse momentum, which is
non-negligible compared with $R_{\left\langle p_{\mathrm{T}}\right\rangle}-1$.
