When I monitor 'BusyWorkerThread' and 'WorkerQueueSize', I could see 'WorkerQueueSize' value is not 0 in spite of 'BusyWorkerThread' did not reach to 'task-max-threads'.

BusyWorkerThreadCount: This shows the number of busy threads in the task worker thread pool. This is available since EAP 7.1.0 and EAP 7.0.9 or later.
WorkerQueueSize: This shows how many counts are awaiting for free worker thread in workQueue. This can be used for monitoring in order to know if the worker thread are full or not.

I set task-max-threads="300" and monitor with jconsole.
I attached the jconsole screen-shot. (0079.jpg, 0081.jpg)