The World Statistics are a little bit borked because there is little validation done of the records submitted (just look at the tens of thousands of "average" scientists).
While it's impossible to determine how this statistic is calculated without delving into the code (do Rockets count as shots? Is Rapid Fire one "shot" or "two"), it is possible to do More than this in a single shot:
A heavy with HEAT ammo shooting at a Shredded target, for instance, can easily do upwards of 30 damage. Plasma Snipers and Crit-Specced Assaults can also break into the 20s, if properly set up.
So while my bet is on mods for many of the "outlier" statistics, it is certainly plausible for average damage to be that high, especially if Rockets count as a single shot.