Batch not seeming to scale down running instances when last few tasks are running

[LinearParadox1]

So I'm running a batch on some workloads that process for a few hours. There are around 50 such tasks, each is independent. I'm finding that when I'm running these tasks in parallel, when I'm at the tail end of my workload, I tend to get some instances that are just doing nothing. 

For example, I have 6 tasks running, with 44 completed.  However, I have 8 instances running (my quota maximum). To me, it seems that these should be shutdown, as there is no more tasks to assign to them. But this does not seem to be the case.