For words that have only three letters, there isn't a 4th letter. For instance with the word “add”, there aren't any much more letters Later on and thus should be the word "insert". The word “addict” is while in the BIP39 wordlist however, you would have utilized “addi” for “addict”. https://doctorbookmark.com/story19079375/5-essential-elements-for-bip39-explained