You unknowingly used a hero save bug.
The hero save glitch has been mentioned many times, I've not been one to use it (on purpose), but I'll explain my understanding.
Basically, when you enter a quest area, the game makes a save. Now, you might have realised, while in the quest area, if you do a save, then load it, you go back to the start of the quest and get to do it again.
However, two things happen:
1) All items/experience you gained from when you started the quest until when you saved and loaded you keep
2) The maps encountered during the quest are reset back to what they were like before you began the quest.
Now, this means you can potentially get 30 silver keys from just one area... however, using the hero save glitch extensively has been known to cause problems within the game, and in my personal opinion, breaks it slightly.
Hope you understand all this.
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