For words that only have 3 letters, there is no 4th letter. For example with the word “add”, there isn't any more letters Later on and so have to be the word "add". The word “addict” is during the BIP39 wordlist however , you would've applied “addi” for “addict”. Try https://socialeweb.com/story4877957/the-basic-principles-of-bip39-passphrase-generator