What is the logic for allocating load to parallel workers in fusion assets?

Summary

While setting the profile FA_NUM_PARALLEL_REQUESTS, we expect the load to be equally spread across the number of child requests. But we find it is not uniform and not predictable.

Content

We are processing 40000 Adjustment through FBDI.

FA_NUM_PARALLEL_REQUESTS is set to 10, hence "Post Mass Fixed Assets Financial Transactions" ten ESS are triggered. Most of them completes immediately as the number of records assigned to the are less. One of the request takes a long duration as it is loaded with 40% of records.

Tweaking the profile FA_BATCH_SIZE did not help. 

Is there a way to make the program evenly distribute the load, so that the process completes quickly.