For words that have only three letters, there's no 4th letter. For instance with the word “add”, there isn't any much more letters Later on and thus need to be the word "increase". The word “addict” is during the BIP39 wordlist however, you would've employed “addi” for “addict”. A seed https://bookmarknap.com/story9033655/facts-about-bip39-seed-phrase-github-revealed