For words that have only 3 letters, there's no 4th letter. For instance with the word “insert”, there won't be any more letters Later on and therefore has to be the word "insert". The word “addict” is while in the BIP39 wordlist however you might have employed “addi” for “addict”. https://social-medialink.com/story4091349/fascination-about-bip39-english-words