Do you have a complete collection?

As far as I know because there are so many users with a complete set it randomly selects 10 people to pay out. If you're checking the high score page after being awarded then it won't show your name because it's already shuffled around (I can't remember if it 'updates' every couple of hours or every day).

If you got it more than once in a row you likely just had some good luck. :P