For words that have only 3 letters, there is no 4th letter. For instance with the word “increase”, there won't be any a lot more letters Later on and thus needs to be the word "include". The word “addict” is inside the BIP39 wordlist but you would have applied “addi” http://remingtonjszi611.almoheet-travel.com/from-words-to-wide-range-how-to-efficiently-use-bip39-passphrases-for-your-copyright-wallets