this is really puzzling...

[kien]

Sometimes it is just luck. There is that element in this hobby as well. Marine life in general can be fragile and unpredictable (especially invertebrates). Powder Blue tangs are difficult to keep and many people are quite unsuccessful with them, but you have one :-). Starfish as well are notoriously hard to keep. For a lot of starfish scientists don't even know what they eat!

Bottom line is don't get discouraged. Sometimes you will bring something home that is supposed to be easy to keep but just ends up dieing and sometimes you'll get lucky and cam keep something that is supposed to be difficult. All else being equal, that is, if your parameters are in check and you're maintaining everything, there are elements that are just out of our control.