A late reply, but HSV keeps too supplies of rental cars, non-EA and EA. So if someone booked a non-EA reservation, they will have a pre-assigned car.

My guess is that she could not find the key for the car you were looking for on/in the key hook / drawer, which would imply they were inside the airport at the non-EA booth. Therefore, she would have assumed/deduced the car was "pre-assigned"

Sometimes if they are not to busy, they can call the non-EA booth and check for sure it was already assigned. If not they give you the car, but you will have to go back inside and get the key.