For words that have only three letters, there isn't a 4th letter. For instance with the word “add”, there are no more letters Later on and therefore have to be the word "increase". The word “addict” is in the BIP39 wordlist however , you might have employed “addi” for “addict”. http://edgargvkh682.theburnward.com/from-mnemonics-to-security-understanding-bip39-passphrases-and-their-importance-in-copyright