the chest is supposed to be a 50% chance without the diamond armlet and a 100% chance with it... per a game mechanics guide...

Or perhaps it was 100/100 and zodiac E shield was 50/100

Perhaps you are at the wrong chest?
If I remember right that was a double chest, meaning you open one chest and there is a "chance" of the excalibur chest being inside/on top of that chest, so you open two of them in that room.. The first is gil/nothing.. the second is excalibur.

Once the chest spawns, there should be little trouble getting the excalibur... its getting that second chest to spawn inside/on top of the first, i think it has a 20% chance to spawn the chest.