For words that have only three letters, there isn't any 4th letter. For example with the word “add”, there isn't any far more letters Later on and therefore have to be the word "add". The word “addict” is during the BIP39 wordlist however, you might have applied “addi” for “addict”. https://bookmarkinginfo.com/story18792432/the-basic-principles-of-bip39-passphrase-generator