The Addon Manager is surely not confusing anything. If it prompts to activate the QW757, it's because you have a serial number in the registry for it, which hasn't been activated yet.
It's not possible this serial number could have ended in your registry other than having manually added, since the Addon Manager won't store a Serial Number in the registry until *after* it has been successfully activated but, since you said you don't own the 757, that serial number must have been added outside the Addon Manager.
The message will go away if you clear up your registry keys for the QW757. Just be careful not to touch anything related to the 787, of course.