I asked about this on my 2006 Statendam, Asia, cruise. We were lucky enough to be at the Captain's table for the duration of this cruise. I asked the Hotel Manager, Deidrik Van Regermorter, where the additional amount goes.
He responded that this money is distributed to non-tipped, lower paid, service staff who also make your cruise more enjoyable. He specifically mentioned the dishwashers; entry level waiters in the buffet and the laundry.
He said that when the final distribution is made it isn't a large amount of money for these people but it does give them added compensation. He did not give me a dollar amount.
I personally doubt that after the distribution anyone would receive more than $25 - $50 per month but it's better than nothing and it's nice to know that the behind the scenes, lower paid, crew receive something.