For words that only have 3 letters, there isn't a 4th letter. For instance with the word “insert”, there won't be any far more letters afterwards and thus needs to be the word "increase". The word “addict” is from the BIP39 wordlist but you would have used “addi” for “addict”. https://bookmarkcitizen.com/story19038711/5-essential-elements-for-bip39-explained