Board Thread:General Discussion/@comment-28059885-20160326095054/@comment-24776254-20160328035757

The chests have a predetermined cycle, so it is not up to chance. You get the Magical chest on the 20th, 140th and 212th chest win. Once you get up to 248 chest wins (so wins where you acquire a chest)  the cycle starts over. If you play and win when your chest slots are full, the chest cycle will NOT progress.

I hope that helps.