Open is not the same thing as Transparent.
In some currencies, the rules may require complete transparency. In others, there may be privacy (of participants, contents of transactions or whatever).
Think of it this way. There are card games where some cards are public and dealt face-up and some are private and dealt face-down. All the players can see that cards were dealt even if they can’t see what the cards are.
Open Data allows all players to see a transaction took place, but the transaction can contain private encrypted data (similar a face-down card). The rules of the currency determine how much transparency there is.
A currency could have numbered accounts where the identity of the people transacting is private (like Swiss bank accounts), the amounts are private or what was transacted was private, but if it shares its data using our Open Data protocols, each transaction will be visible, but what parts of that transaction are readable without special keys is customizable.