For words that have only three letters, there isn't a 4th letter. For instance with the word “add”, there are no extra letters Later on and so has to be the word "include". The word “addict” is within the BIP39 wordlist but you would've made use of “addi” for “addict”. https://bookmarkspiral.com/story19054095/little-known-facts-about-bip39-words