Glass is tough on bullets, and plays hell with the trajectory. Shooting through glass changes the path of the bullet, sometimes substantially, and in only somewhat predictable ways. A pro would compensate for the expected trajectory deviation caused by the glass, but even this isn't a sure thing. An amateur who doesn't know how to compensate his point of aim for the glass will very likely blow the shot unless he gets lucky.
Yep, as the shooter in this incident may have been...IF the hit on the glass represents one of two shots. On a nice day, with clean glass, a display window at a distance can create a reflection which not only seems solid, it also appears to have depth.