[otrs] default agent list per queue

We are using OTRS 2.3.4 with ITSM 1.3 installed. I see that on many of our queues there will be 2 or 3 agents (out of a list of about 70) that automatically appear in the short list for the owner selection field (in the dropdown list) when creating a new incident ticket. Does anyone know where this is controlled or how to modify the short list? We can click on -All- and see the entire list, but I would like to be able to control which agents appear in the short list for each queue. Thanks, Pat