For words that only have 3 letters, there's no 4th letter. For instance with the word “insert”, there are no extra letters Later on and therefore has to be the word "include". The word “addict” is in the BIP39 wordlist however you would have utilized “addi” for “addict”. This additional https://followbookmarks.com/story19240883/5-essential-elements-for-bip39-explained