  By deriving a general expression for multiplicity distribution (a conditional
probability distribution) in statistical model, we demonstrate the mismatches
between experimental measurements and previous theoretical calculations on
multiplicity fluctuations. From the corrected formula, we develop an improved
baseline measure for multiplicity distribution under Poisson approximation in
statistical model to replace the traditional Poisson expectations. We find that
the ratio of the mean multiplicity to the corresponding reference multiplicity
are crucial to systemically explaining the measured scale variances of total
charge distributions in different experiments, as well as understanding the
centrality resolution effect observed in experiment. The improved statistical
expectations, albeit simple, work well in describing the negative binomial
multiplicity distribution measured in experiments, e.g. the cumulants (cumulant
products) of total (net) electric charge distributions.
