I agree that finding out the reason why they are handing them in so early makes sense. They may just like to clear their to do sheet and not be looking to take on more.
If there is a practical element to the task, then overachieving may not be beneficial.
Sometimes we need to be able to show a level of consistency in what we are doing and consolidate our learning. In this case I would be looking to slow them down a little.
I feel it would depend on the type of study they were doing, are they working towards a qualification of some form or using the information in a work setting?
(Ruth)