For words that only have three letters, there is not any 4th letter. For instance with the word “add”, there aren't any much more letters Later on and therefore should be the word "insert". The word “addict” is in the BIP39 wordlist but you would have employed “addi” for “addict”. https://bookmarkdistrict.com/story18576290/a-review-of-bip39-phrase-list