For words that have only three letters, there isn't any 4th letter. For instance with the word “add”, there are no additional letters Later on and so need to be the word "insert". The word “addict” is from the BIP39 wordlist however, you would have utilised “addi” for “addict”. In https://bookmarkpath.com/story19569651/5-essential-elements-for-bip39-explained