The disk encryption password is separate from the user login. It has to be, if you consider that the device may have more than one user [0]. [0] which is rare

The disk encryption password is separate from the user login. It has to be, if you consider that the device may have more than one user [0]. [0] which is rare bordering on "never happens", but since contemporary desktop OSs are all derived from multi-user server OSs that's what we got.