My understanding, from what I have read about this type of ammo, is that it has a faster burn rate, therefore, more of the powder will burn before the bullet exits the barrel. Rifle ammo has slower burn rates due to the longer barrel. The powder should still be burning, and therefore, gases expanding to propel the bullet until the bullet exits the barrel. If not, the bullet would slow down before exiting the barrel. The physics behind burn rates, powder formulas, powder sizes and shapes and barrel designs and lengths is quite interesting.
Unburned powder or burned after exiting the barrel is wasted energy not used to propel the bullet, which reduces bullet speed. Regular ammo does not attain the advertised velocities in short-barreled handguns because of that. Manufacturers have now found a new niche market for concealed carry with this ammo that we are willing to pay extra for. Previously it was hollow-point, then filled hollow-points (legal in New Jersey); frangible; pressed metal powder bullets, and now cartridges for short-barreled handguns.