I didn't go through every page to see if this was posted. But my theory is they are creating a pool to make sure there is no wastage in visa number. On the other hand everyone who gets an EAD has to be renew it every 1 or 2 years and hence keeps the renewal fee coming.
Good Source of funding.
They did it for last 5 years since 2007 and continue to do that for people in EB3 IC.
Just my 2 cents.