This is because of the Transport Modes of the different Responders.
Beacon will allow at least one vehicle capable of patient transport (e.g., ambulance, car, boat) to be assigned to each incident when you broadcast alerts. So, if you requested a maximum of one responder, and the first one to reply was on foot, if a car replied after, Beacon would still accept it.
This is an artifact of having originally designed Beacon exclusively for ambulance services and fire departments. In a future v5.0, it will be resolved.