For words that have only 3 letters, there isn't a 4th letter. For instance with the word “add”, there won't be any extra letters Later on and therefore need to be the word "insert". The word “addict” is from the BIP39 wordlist however , you would've used “addi” for “addict”. https://bookmark-dofollow.com/story21758958/fascination-about-bip39-english-words