I have seen this numerous times, in different airports. Screen says yes, someone comes up and asks and the GA says no.
Sometimes it's that the GA is taking volunteers and already has enough on the list to feel comfortable, sometimes it's just that they don't need any.
I would say that the trigger is clearly more conservative than the average GA's risk tolerance, and/or that it's not good at making judgement calls about misconnects, etc. Regardless, it seems to be decoupled from the GAs.