For words that have only three letters, there is absolutely no 4th letter. For instance with the word “add”, there aren't any much more letters Later on and therefore needs to be the word "insert". The word “addict” is within the BIP39 wordlist but you would've applied “addi” for “addict”. https://bookmarkfly.com/story18971269/bip39-phrase-generator-can-be-fun-for-anyone