Petals Around the Rose

Kotaku posted an article about Mensa teaming with Empire Interactive on making a new game(s). One of the comments had a link to an old Mensa game that's rather cute once you get the hang of it. It's not terribly difficult, but can be tricky in the wrong mentality. It's called Petals Around the Rose and can be found here.

Math puzzle

When does 10 - 11 = 11?

Answer in the comments. First to get it right gets +1 Internets.