For words that have only 3 letters, there is not any 4th letter. For example with the word “insert”, there aren't any additional letters afterwards and so has to be the word "add". The word “addict” is while in the BIP39 wordlist but you might have used “addi” for “addict”. https://thebookmarkplaza.com/story18781276/a-secret-weapon-for-bip39-words-to-numbers