I was offered the 10,000 points for 7 nights. I completed my 7th night earlier this month and the 10,000 points posted right about the same time as my stay credit.

I just took a look at my account and clicked on the MegaBonus promotion link for fun. It now says 15,000 points for 7 nights. I then flipped back to look at my account to see if I got 10K or 15K. My account showed 10K. Then I flipped back to the mega bonus page and it now says 10 nights for 20,000 points.

Perhaps once you have reached your threshold, they then raise the bar and see if you will try to cram in more nights with just a few weeks left in the promotion.