For words that only have 3 letters, there isn't a 4th letter. One example is with the word “increase”, there aren't any more letters Later on and thus have to be the word "add". The word “addict” is while in the BIP39 wordlist but you would've utilised “addi” for “addict”. https://esocialmall.com/story5022880/not-known-factual-statements-about-bip39-explained