For words that have only three letters, there isn't a 4th letter. One example is with the word “add”, there isn't any extra letters Later on and so have to be the word "incorporate". The word “addict” is within the BIP39 wordlist however you would have utilised “addi” for “addict”. https://opensocialfactory.com/story19301395/the-invalid-bip39-phrase-diaries