My guess would be that the 12 is already near the limit. The two "free" stars feel like a little personal gift, especially if they actually punch them out when they give them to you, with a sly wink or something. If it was 900/1000, it would feel confusing and dishonest.

You might also gain some actual loyalty if you give them the last one for free, since that would be an unexpected treat reserved for people who bought 9 cups of coffee.