Treasure Chest Opened After Watching Ad

I watched an ad and opened it. It was an old treasure chest in the Elf Forest, and I got a 5000 EXP Potion. It was a 28% chance, but it was a dud. Old Treasure Chest in Elf Forest Item Chance 500 Coins 0.1% 3,000 Gems 1.5% 5 Gem Boosters: 1 hour 1.6% 5 Gem Boosters: 30 min 2.5% 1,000 Gems 3% 500 Gems 5% 10,000 EXP Up 18% 5,000 EXP Up 28% 3,000 EXP Up 40.3%