When a task can be run on more than one Server and a Preferred Server is not specified then load balancing will come into effect.
Each Server waits for an amount of time before attempting to allocate itself to run a task.
How long it waits, i.e. how eager it is will be calculated based on:-
it's Priority, how many tasks it is currently running and it's thread count.
These values can be found in the table APP.MACHINES in the fields THREADCOUNT,TASKCOUNT and PRIORITY
The server priority is set in the presenceconfig.xml and read when the server starts up.
Note, as stated above, if the task has excluded servers or a preferred server then Load balancing will take a back seat.
Right clicking on a Task in the Task Explorer will show the following screen where the aforementioned settings can be made.