For words that have only three letters, there isn't a 4th letter. For instance with the word “insert”, there won't be any a lot more letters afterwards and thus must be the word "increase". The word “addict” is from the BIP39 wordlist however, you would have utilized “addi” for “addict”. https://bookmarkity.com/story19067035/5-essential-elements-for-bip39-explained