5% fee is on conversion from Hive to HBD, not on internal exchange.

Internal exchange is when you place an order and wait for it to be matched with opposite order from other people. Your existing Hive is sent to the other guy and his existing HBD is sent to you.

On the other hand you can convert Hive to HBD without involving other people, however you pay 5% fee for it (just Hive-to-HBD, HBD-to-Hive conversion has no fee). In the process you burn Hive and print new HBD. Their respective supplies change.