It's a little bit confusing, I agree.
My first expansion I too sat there wondering if I've made a mistake or if they gave me a wrong key.

But like people here has already mentioned, the key you registred was only the Pre-Order/Early Access key.
The core game key will be sent/unlocked (which will be the version you bought) in your order at the store around the launch date of the expansion.
And by then you will have to registre that key as well on your account.
Or else you won't be able to gain access to the new expansion once it has released.