For words that have only three letters, there isn't a 4th letter. One example is with the word “insert”, there are no a lot more letters afterwards and thus needs to be the word "include". The word “addict” is within the BIP39 wordlist but you would've used “addi” for “addict”. https://orangebookmarks.com/story19088326/not-known-facts-about-bip39-english-words