If you tell me that you won't be home before 6, I have information about the period up to 6, but not after. I know you won't be home before 6 but I don't know when you will be home. Telling me you won't be home until 6 gives the information up to 6 and also after. I know that at 6, you'll come home. 'Until' finishes 'won't be home'.