Well i would say its like this:
Orginal DVD5 Game -> WiiKey . o (Its orginal lets turn myself off) -> Getting accepted by the Wii
Backup DVD5 Game -> WiiKey . o (Its a backup lets patch) -> Getting acepted on the Wii
Orginal DVD9 Game -> WiiKey . o (Whats this... looks like a backup lets patch) -> The Wii dont accepts the disk
Backup DVD9 Game - > WiiKey . o (Its a backup lets patch) -> Getting accpeted by the Wii.
Thats how i think its working.
The WiiKey propperly cannot detect that the Orginal DVD9 game is orginal and tries to patch legit code to unlegit code.