For words that have only 3 letters, there's no 4th letter. For example with the word “insert”, there aren't any more letters Later on and thus have to be the word "include". The word “addict” is during the BIP39 wordlist however you would've employed “addi” for “addict”. This text clarifies https://linkingbookmark.com/story18702583/bip39-seed-phrase-fundamentals-explained