Octopus card

Today, because my octopus card, which is an electric card that can be used in public transport as a fare, insteading of inserting coins, notes or using ticket when you take a train or buses , is running out of money, so I went to MTR station to refill my card.
There are two people before me as I jump a queue for the counter. They look like a couple and they are foreigner. I have heard the conversation of the man and the staff. It is what I have heard it.

[ I want to refund this card please . Because I will be flying to oversea within three days and I will take a long time to stay there. I think this card is not necessary now. ] the man said.

[ I understand, sir. Please wait for a moment. ] the staff said.

After a few minutes later........

[ Sir, the card is cancelled now. The refund is 30 dollars .] the staff said.

[ It is odd. Why the refund is 30 dollars, but not 50 dollars ? 
The deposit for this card is 50 dollars when I used it in a very       
first time. And the staff said I can take a full refund for this card if I don't use it and refund it. ] the man said curiously.

[ Sir, as I see the record of this card, the last time you used it is yesterday, and the rest of saving in your card display 10    
dollars at that moment. The last time you used it by taking a train for 40 dollars . Because it is not enough money to pay the fare for just 10 dollars, the deposit will be used as a compensation automatically for the first time. Then, the card will be deactivated unless it is refilled and the remain is positive. ] the staff explained.

[ oh, I see. Thank you very much.....] the man said thankfully.

[ You are welcom. Have a nice day, sir.] the staff said with a smile.