For words that have only three letters, there is absolutely no 4th letter. For example with the word “add”, there are no much more letters Later on and thus need to be the word "insert". The word “addict” is within the BIP39 wordlist however, you would've utilised “addi” for “addict”. https://thesocialvibes.com/story4642306/not-known-factual-statements-about-bip39-explained