I was playing Angry Birds offline, and now when I'm back online and signed into my Rovio Account, my offline progress has been lost. Why is that?

Scores are only saved to your Rovio Account when you are playing signed into Rovio Account. You can sign into Rovio Account when online and then go offline. Your scores will then be saved to Rovio Account when you are online again. When playing offline and signed out of Rovio Account your scores are still saved locally on a device but then when you sign back into your Rovio Account the scores will revert back to the last time you played online and signed into your Rovio Account.