Javadocs for DistributedExecutionService:
Execution environment will chose an arbitrary node N hosting some or all of the keys specified as input. If all keys are not available locally at node N they will be retrieved from the cluster.
If you want to check the implementation, just look into DefaultExecutionService. You'll find that it really grabs all nodes that own at least one of those keys, and then randomly picks one of them to execute the task.