Drills and real emergencies are clearly different. There is no way of telling when or what will happen in a real emergency. On the other hand, drills are typically planned and a perfect time to practice the EAP including accounting for everyone as safely evacuated.
The problem with drills is that they are often not taken seriously. There are employees who do not listen or pay attention during the evacuation process. There are employees who disappear and go off site. There are employees who ignore the drills and remain in their offices because they view their work as more important than the drill. Unfortunately, in too many cases, drills are not taken seriously due to the lack of real danger. This is a problem because when there is an actual emergency, employees will not know what to do. Companies have to mandate that employees take drills seriously. Drills should be mandatory, practiced regularly, metrics should be collected, and the EAP should be evaluated for improvements.