This is from my son who is an acoustical engineer who has done work for firing ranges on sound mitigation projects:  Firing a live round would have two acoustic components - the expansion of the gas from the cartridge, and the bullet itself breaking the sound barrier, while a blank round would only have the former.  Also, with a live round, the expanding gas is constrained by the round until it leaves the barrel, thus raising the pressure (making it louder), as compared to an open barrel with the blank.  I would guess that the sound signature and intensity are different enough to be picked up by the system, and looking at their website, that kind of classification is built in to give police a better idea of what may be occurring (you wouldn't want the SWAT team called in for a backfiring car, for instance).