For words that have only 3 letters, there isn't a 4th letter. For instance with the word “add”, there are no extra letters Later on and therefore needs to be the word "insert". The word “addict” is within the BIP39 wordlist but you would've employed “addi” for “addict”. An easy https://bookmark-share.com/story19036796/an-unbiased-view-of-bip39-seed-phrase-github