For words that have only three letters, there's no 4th letter. For example with the word “add”, there aren't any extra letters Later on and thus needs to be the word "incorporate". The word “addict” is inside the BIP39 wordlist however , you would've applied “addi” for “addict”. On this https://topsocialplan.com/story4353805/the-ultimate-guide-to-bip39-seed-phrase-github