There is a display error on the event when using the app. You are not supposed to get 10 stars when you press the start (or shuffle) instead you get the 10 daily stars from collecting from town hall. The ten from pressing the start are removed automatically and do not register within the system. So when you see -20, that's the -10 for picking the present, and the additional 10 that should not have been added being taken away again.
This issue has been forwarded to our development team for fixing.