Sometimes during a call to the control room it is not made clear that a cardiac arrest is involved. AED and resuscitation alerts are only sent out when an ambulance is sent for (assistance) resuscitation.
It may be the case that on the way to the incident, or at the incident, the alert is switched to resuscitation. In that case, a second ambulance is sent for assistance resuscitation. At that moment, first responders are also called in to provide assistance.
By that time, the first ambulance could already be (nearly) at the incident, making you think assistance is no longer necessary. However, assistance is still desired, even when you can hear the siren or see the ambulance already. When the first ambulance has arrived, you can sometimes really help the ambulance staff. Should this not be necessary, the first responders present will make this clear.