My logic says : (I MUST be wrong, just guessing)

1. Card : Stores card serial number, credit balance (bcoz you can check balance with SmartTag)
2. Server (database) : Stores credit balance.

Only place to reload the card will definitely go through their server, so server sure got latest credit balance and information. Even if you tweak the card balance to "9999999", in database your balance still 0 also useless bcoz when you use your card, for sure they will check your balance through database.

As for the reader at any tng device definitely is a read/write reader. Bcoz they need to write balance into the card, and read the card serial number to get your balance info.