For words that only have three letters, there is not any 4th letter. One example is with the word “add”, there isn't any extra letters Later on and thus need to be the word "include". The word “addict” is during the BIP39 wordlist however you would've employed “addi” for “addict”. https://indexedbookmarks.com/story18736536/5-simple-techniques-for-bip39-seed-phrase