For words that only have 3 letters, there isn't a 4th letter. For instance with the word “include”, there isn't any additional letters Later on and thus needs to be the word "increase". The word “addict” is while in the BIP39 wordlist but you would've utilised “addi” for “addict”. You https://infopagex.com/story4158590/not-known-factual-statements-about-bip39-explained