I feel your pain... I'm still waiting on my chestpiece. I've done countless False Emperor and I have yet to receive my chest.
As for the 25% thing, I'm not too sure on the percentage but it's a global roll so everyone that has completed it has a ___% chance to receive that item. So one team gets it and the next 3 don't and the next team does, but then the next 6 dont and the next 2 after that do. It's a very complicated process.