For words that only have three letters, there isn't a 4th letter. For instance with the word “include”, there are no additional letters Later on and therefore should be the word "add". The word “addict” is during the BIP39 wordlist however you would have made use of “addi” for “addict”. https://bookmarkcork.com/story20105739/new-step-by-step-map-for-bip39-english-words