Why wouldn't they test before the deadline? At £200 a per person, it's in their interest as a business to have as many people assessed as possible.
Waiting until the closing date of the application window will not prevent them from assessing all who meet the minimum entry requirements, and therefore it makes no difference to the amount they will earn. I suspect it has far more to do with logistical and operational reasons than anything else.
But holding an assessment day before receiving all applications does seem a peculiar approach. I imagine news of it probably encouraged some people to rush their applications, which will surely disadvantage them. Rather discombobulating if you ask me.