Special Chests (Super Object) vs. Type I Ordinary Objects (how do they differ?)
The following explanation is paraphrased from a post by Jarulf...
First, the game will check if the chest is locked, and if so, unlock it if you have a key or are an assassin.
Second, the game sets the number of drop cycles from the appropriate Chest TC to 2 if locked (up to 8 item drops), otherwise to 1 (up to 4 item drops).
Next, the game tests for a flag in the initialization routine. Special Chests always have this flag set. If it is set, there is a 5% chance that the game will
make the desired quality of the drops rare, otherwise (95% chance) the desired quality will be magic. If the flag is not set
there is no desired quality and the game will pick quality normally, applying MF and so on (Type I Ordinary Objects).
After that, the game will make a random chance to drop at all. 75% of the time it will drop. The other 25% of the time, it will only drop
if the flag is set or if the chest was locked.
Next, the game will make either 1 or 2 actual Chest TC drop cycles (depending on if it is locked or not), with a possible desired quality as mentioned above.
If the object is a Special Chest AND none of the previous Chest TC drop cycles (1 or 2) passed a magical+ test, it will make up to 10 more Chest TC drop cycles with a possible desired quality as mentioned above. As soon as 1 of them passes a magical+ test, the 10 attempts are aborted.