For words that have only 3 letters, there isn't a 4th letter. Such as with the word “add”, there aren't any far more letters Later on and therefore need to be the word "increase". The word “addict” is in the BIP39 wordlist but you would have used “addi” for “addict”. https://trackbookmark.com/story21262935/not-known-facts-about-bip39-english-words